A simple framework for contrastive learning of visual representations

A Simple Framework for Contrastive Learning of Visual Representations
Ting Chen 1
Simon Kornblith 1
Mohammad Norouzi 1
Geoffrey Hinton 1
Abstract
This paper presents SimCLR: a simple framework
for contrastive learning of visual representations.
We simplify recently proposed contrastive self-
supervised learning algorithms without requiring
specialized architectures or a memory bank. In
order to understand what enables the contrastive
prediction tasks to learn useful representations,
we systematically study the major components of
our framework. We show that (1) composition of
data augmentations plays a critical role in defining
effective predictive tasks, (2) introducing a learn-
able nonlinear transformation between the repre-
sentation and the contrastive loss substantially im-
proves the quality of the learned representations,
and (3) contrastive learning benefits from larger
batch sizes and more training steps compared to
supervised learning. By combining these findings,
we are able to considerably outperform previous
methods for self-supervised and semi-supervised
learning on ImageNet. A linear classifier trained
on self-supervised representations learned by Sim-
CLR achieves 76.5% top-1 accuracy, which is a
7% relative improvement over previous state-of-
the-art, matching the performance of a supervised
ResNet-50. When fine-tuned on only 1% of the
labels, we achieve 85.8% top-5 accuracy, outper-
forming AlexNet with 100× fewer labels. 1
1. Introduction
Learning effective visual representations without human
supervision is a long-standing problem. Most mainstream
approaches fall into one of two classes: generative or dis-
criminative. Generative approaches learn to generate or
otherwise model pixels in the input space (Hinton et al.,
2006; Kingma & Welling, 2013; Goodfellow et al., 2014).
1
Google Research, Brain Team. Correspondence to: Ting Chen
<iamtingchen@google.com>.
Proceedings of the 37th
International Conference on Machine
Learning, Vienna, Austria, PMLR 119, 2020. Copyright 2020 by
the author(s).
1
Code available at https://github.com/google-research/simclr.
25 50 100 200 400 626
Number of Parameters (Millions)
55
60
65
70
75
ImageNet
Top-1
Accuracy
(%) InstDisc
Rotation
BigBiGAN
LA
CPCv2
CPCv2-L
CMC
AMDIM
MoCo
MoCo (2x)
MoCo (4x)
PIRL
PIRL-ens.
PIRL-c2x
SimCLR
SimCLR (2x)
SimCLR (4x)
Supervised
Figure 1. ImageNet Top-1 accuracy of linear classifiers trained
on representations learned with different self-supervised meth-
ods (pretrained on ImageNet). Gray cross indicates supervised
ResNet-50. Our method, SimCLR, is shown in bold.
However, pixel-level generation is computationally expen-
sive and may not be necessary for representation learning.
Discriminative approaches learn representations using objec-
tive functions similar to those used for supervised learning,
but train networks to perform pretext tasks where both the in-
puts and labels are derived from an unlabeled dataset. Many
such approaches have relied on heuristics to design pretext
tasks (Doersch et al., 2015; Zhang et al., 2016; Noroozi &
Favaro, 2016; Gidaris et al., 2018), which could limit the
generality of the learned representations. Discriminative
approaches based on contrastive learning in the latent space
have recently shown great promise, achieving state-of-the-
art results (Hadsell et al., 2006; Dosovitskiy et al., 2014;
Oord et al., 2018; Bachman et al., 2019).
In this work, we introduce a simple framework for con-
trastive learning of visual representations, which we call
SimCLR. Not only does SimCLR outperform previous work
(Figure 1), but it is also simpler, requiring neither special-
ized architectures (Bachman et al., 2019; Hénaff et al., 2019)
nor a memory bank (Wu et al., 2018; Tian et al., 2019; He
et al., 2019; Misra & van der Maaten, 2019).
In order to understand what enables good contrastive repre-
sentation learning, we systematically study the major com-
ponents of our framework and show that:
arXiv:2002.05709v3
[cs.LG]
1
Jul
2020

• Composition of multiple data augmentation operations
is crucial in defining the contrastive prediction tasks that
yield effective representations. In addition, unsupervised
contrastive learning benefits from stronger data augmen-
tation than supervised learning.
• Introducing a learnable nonlinear transformation be-
tween the representation and the contrastive loss substan-
tially improves the quality of the learned representations.
• Representation learning with contrastive cross entropy
loss benefits from normalized embeddings and an appro-
priately adjusted temperature parameter.
• Contrastive learning benefits from larger batch sizes and
longer training compared to its supervised counterpart.
Like supervised learning, contrastive learning benefits
from deeper and wider networks.
We combine these findings to achieve a new state-of-the-art
in self-supervised and semi-supervised learning on Ima-
geNet ILSVRC-2012 (Russakovsky et al., 2015). Under the
linear evaluation protocol, SimCLR achieves 76.5% top-1
accuracy, which is a 7% relative improvement over previous
state-of-the-art (Hénaff et al., 2019). When fine-tuned with
only 1% of the ImageNet labels, SimCLR achieves 85.8%
top-5 accuracy, a relative improvement of 10% (Hénaff et al.,
2019). When fine-tuned on other natural image classifica-
tion datasets, SimCLR performs on par with or better than
a strong supervised baseline (Kornblith et al., 2019) on 10
out of 12 datasets.
2. Method
2.1. The Contrastive Learning Framework
Inspired by recent contrastive learning algorithms (see Sec-
tion 7 for an overview), SimCLR learns representations
by maximizing agreement between differently augmented
views of the same data example via a contrastive loss in
the latent space. As illustrated in Figure 2, this framework
comprises the following four major components.
• A stochastic data augmentation module that transforms
any given data example randomly resulting in two cor-
related views of the same example, denoted x̃i and x̃j,
which we consider as a positive pair. In this work, we
sequentially apply three simple augmentations: random
cropping followed by resize back to the original size, ran-
dom color distortions, and random Gaussian blur. As
shown in Section 3, the combination of random crop and
color distortion is crucial to achieve a good performance.
• A neural network base encoder f(·) that extracts repre-
sentation vectors from augmented data examples. Our
framework allows various choices of the network archi-
tecture without any constraints. We opt for simplicity
and adopt the commonly used ResNet (He et al., 2016)
←− Representation −→
x
x̃i x̃j
hi hj
zi zj
t ∼ T
t
0 ∼ T
f(·) f(·)
g(·) g(·)
Maximize agreement
Figure 2. A simple framework for contrastive learning of visual
representations. Two separate data augmentation operators are
sampled from the same family of augmentations (t ∼ T and
t0
∼ T ) and applied to each data example to obtain two correlated
views. A base encoder network f(·) and a projection head g(·)
are trained to maximize agreement using a contrastive loss. After
training is completed, we throw away the projection head g(·) and
use encoder f(·) and representation h for downstream tasks.
to obtain hi = f(x̃i) = ResNet(x̃i) where hi ∈ Rd
is
the output after the average pooling layer.
• A small neural network projection head g(·) that maps
representations to the space where contrastive loss is
applied. We use a MLP with one hidden layer to obtain
zi = g(hi) = W(2)
σ(W(1)
hi) where σ is a ReLU non-
linearity. As shown in section 4, we find it beneficial to
define the contrastive loss on zi’s rather than hi’s.
• A contrastive loss function defined for a contrastive pre-
diction task. Given a set {x̃k} including a positive pair
of examples x̃i and x̃j, the contrastive prediction task
aims to identify x̃j in {x̃k}k6=i for a given x̃i.
We randomly sample a minibatch of N examples and define
the contrastive prediction task on pairs of augmented exam-
ples derived from the minibatch, resulting in 2N data points.
We do not sample negative examples explicitly. Instead,
given a positive pair, similar to (Chen et al., 2017), we treat
the other 2(N − 1) augmented examples within a minibatch
as negative examples. Let sim(u, v) = u>
v/kukkvk de-
note the dot product between `2 normalized u and v (i.e.
cosine similarity). Then the loss function for a positive pair
of examples (i, j) is defined as
`i,j = − log
exp(sim(zi, zj)/τ)
P2N
k=1 1[k6=i] exp(sim(zi, zk)/τ)
, (1)
where 1[k6=i] ∈ {0, 1} is an indicator function evaluating to
1 iff k 6= i and τ denotes a temperature parameter. The fi-
nal loss is computed across all positive pairs, both (i, j)
and (j, i), in a mini-batch. This loss has been used in
previous work (Sohn, 2016; Wu et al., 2018; Oord et al.,
2018); for convenience, we term it NT-Xent (the normalized
temperature-scaled cross entropy loss).

Algorithm 1 SimCLR’s main learning algorithm.
input: batch size N, constant τ, structure of f, g, T .
for sampled minibatch {xk}N
k=1 do
for all k ∈ {1, . . . , N} do
draw two augmentation functions t∼T , t0
∼T
# the first augmentation
x̃2k−1 = t(xk)
h2k−1 = f(x̃2k−1) # representation
z2k−1 = g(h2k−1) # projection
# the second augmentation
x̃2k = t0
(xk)
h2k = f(x̃2k) # representation
z2k = g(h2k) # projection
end for
for all i ∈ {1, . . . , 2N} and j ∈ {1, . . . , 2N} do
si,j = z>
i zj/(kzikkzjk) # pairwise similarity
end for
define `(i, j) as `(i, j)=− log
exp(si,j /τ)
P2N
k=1 1[k6=i] exp(si,k/τ)
L = 1
2N
PN
k=1 [`(2k−1, 2k) + `(2k, 2k−1)]
update networks f and g to minimize L
end for
return encoder network f(·), and throw away g(·)
Algorithm 1 summarizes the proposed method.
2.2. Training with Large Batch Size
To keep it simple, we do not train the model with a memory
bank (Wu et al., 2018; He et al., 2019). Instead, we vary
the training batch size N from 256 to 8192. A batch size
of 8192 gives us 16382 negative examples per positive pair
from both augmentation views. Training with large batch
size may be unstable when using standard SGD/Momentum
with linear learning rate scaling (Goyal et al., 2017). To
stabilize the training, we use the LARS optimizer (You et al.,
2017) for all batch sizes. We train our model with Cloud
TPUs, using 32 to 128 cores depending on the batch size.2
Global BN. Standard ResNets use batch normaliza-
tion (Ioffe & Szegedy, 2015). In distributed training with
data parallelism, the BN mean and variance are typically
aggregated locally per device. In our contrastive learning,
as positive pairs are computed in the same device, the model
can exploit the local information leakage to improve pre-
diction accuracy without improving representations. We ad-
dress this issue by aggregating BN mean and variance over
all devices during the training. Other approaches include
shuffling data examples across devices (He et al., 2019), or
replacing BN with layer norm (Hénaff et al., 2019).
2
With 128 TPU v3 cores, it takes ∼1.5 hours to train our
ResNet-50 with a batch size of 4096 for 100 epochs.
A B
(a) Global and local views.
C
D
(b) Adjacent views.
Figure 3. Solid rectangles are images, dashed rectangles are ran-
dom crops. By randomly cropping images, we sample contrastive
prediction tasks that include global to local view (B → A) or
adjacent view (D → C) prediction.
2.3. Evaluation Protocol
Here we lay out the protocol for our empirical studies, which
aim to understand different design choices in our framework.
Dataset and Metrics. Most of our study for unsupervised
pretraining (learning encoder network f without labels)
is done using the ImageNet ILSVRC-2012 dataset (Rus-
sakovsky et al., 2015). Some additional pretraining experi-
ments on CIFAR-10 (Krizhevsky & Hinton, 2009) can be
found in Appendix B.9. We also test the pretrained results
on a wide range of datasets for transfer learning. To evalu-
ate the learned representations, we follow the widely used
linear evaluation protocol (Zhang et al., 2016; Oord et al.,
2018; Bachman et al., 2019; Kolesnikov et al., 2019), where
a linear classifier is trained on top of the frozen base net-
work, and test accuracy is used as a proxy for representation
quality. Beyond linear evaluation, we also compare against
state-of-the-art on semi-supervised and transfer learning.
Default setting. Unless otherwise specified, for data aug-
mentation we use random crop and resize (with random
flip), color distortions, and Gaussian blur (for details, see
Appendix A). We use ResNet-50 as the base encoder net-
work, and a 2-layer MLP projection head to project the
representation to a 128-dimensional latent space. As the
loss, we use NT-Xent, optimized using LARS with learning
rate of 4.8 (= 0.3 × BatchSize/256) and weight decay of
10−6
. We train at batch size 4096 for 100 epochs.3
Fur-
thermore, we use linear warmup for the first 10 epochs,
and decay the learning rate with the cosine decay schedule
without restarts (Loshchilov & Hutter, 2016).
3. Data Augmentation for Contrastive
Representation Learning
Data augmentation defines predictive tasks. While data
augmentation has been widely used in both supervised and
unsupervised representation learning (Krizhevsky et al.,
3
Although max performance is not reached in 100 epochs, rea-
sonable results are achieved, allowing fair and efficient ablations.

(a) Original (b) Crop and resize (c) Crop, resize (and flip) (d) Color distort. (drop) (e) Color distort. (jitter)
(f) Rotate {90◦
, 180◦
, 270◦
} (g) Cutout (h) Gaussian noise (i) Gaussian blur (j) Sobel filtering
Figure 4. Illustrations of the studied data augmentation operators. Each augmentation can transform data stochastically with some internal
parameters (e.g. rotation degree, noise level). Note that we only test these operators in ablation, the augmentation policy used to train our
models only includes random crop (with flip and resize), color distortion, and Gaussian blur. (Original image cc-by: Von.grzanka)
2012; Hénaff et al., 2019; Bachman et al., 2019), it has
not been considered as a systematic way to define the con-
trastive prediction task. Many existing approaches define
contrastive prediction tasks by changing the architecture.
For example, Hjelm et al. (2018); Bachman et al. (2019)
achieve global-to-local view prediction via constraining the
receptive field in the network architecture, whereas Oord
et al. (2018); Hénaff et al. (2019) achieve neighboring view
prediction via a fixed image splitting procedure and a con-
text aggregation network. We show that this complexity can
be avoided by performing simple random cropping (with
resizing) of target images, which creates a family of predic-
tive tasks subsuming the above mentioned two, as shown in
Figure 3. This simple design choice conveniently decouples
the predictive task from other components such as the neural
network architecture. Broader contrastive prediction tasks
can be defined by extending the family of augmentations
and composing them stochastically.
3.1. Composition of data augmentation operations is
crucial for learning good representations
To systematically study the impact of data augmentation,
we consider several common augmentations here. One type
of augmentation involves spatial/geometric transformation
of data, such as cropping and resizing (with horizontal
flipping), rotation (Gidaris et al., 2018) and cutout (De-
Vries & Taylor, 2017). The other type of augmentation
involves appearance transformation, such as color distortion
(including color dropping, brightness, contrast, saturation,
hue) (Howard, 2013; Szegedy et al., 2015), Gaussian blur,
and Sobel filtering. Figure 4 visualizes the augmentations
that we study in this work.
Crop
Cutout
Color
Sobel
Noise
Blur
Rotate
Average
2nd transformation
Crop
Cutout
Color
Sobel
Noise
Blur
Rotate
1st
transformation
33.1 33.9 56.3 46.0 39.9 35.0 30.2 39.2
32.2 25.6 33.9 40.0 26.5 25.2 22.4 29.4
55.8 35.5 18.8 21.0 11.4 16.5 20.8 25.7
46.2 40.6 20.9 4.0 9.3 6.2 4.2 18.8
38.8 25.8 7.5 7.6 9.8 9.8 9.6 15.5
35.1 25.2 16.6 5.8 9.7 2.6 6.7 14.5
30.0 22.5 20.7 4.3 9.7 6.5 2.6 13.8
10
20
30
40
50
Figure 5. Linear evaluation (ImageNet top-1 accuracy) under in-
dividual or composition of data augmentations, applied only to
one branch. For all columns but the last, diagonal entries corre-
spond to single transformation, and off-diagonals correspond to
composition of two transformations (applied sequentially). The
last column reflects the average over the row.
To understand the effects of individual data augmentations
and the importance of augmentation composition, we in-
vestigate the performance of our framework when applying
augmentations individually or in pairs. Since ImageNet
images are of different sizes, we always apply crop and re-
size images (Krizhevsky et al., 2012; Szegedy et al., 2015),
which makes it difficult to study other augmentations in
the absence of cropping. To eliminate this confound, we
consider an asymmetric data transformation setting for this
ablation. Specifically, we always first randomly crop im-
ages and resize them to the same resolution, and we then
apply the targeted transformation(s) only to one branch of
the framework in Figure 2, while leaving the other branch
as the identity (i.e. t(xi) = xi). Note that this asymmet-

(a) Without color distortion. (b) With color distortion.
Figure 6. Histograms of pixel intensities (over all channels) for
different crops of two different images (i.e. two rows). The image
for the first row is from Figure 4. All axes have the same range.
Color distortion strength
Methods 1/8 1/4 1/2 1 1 (+Blur) AutoAug
SimCLR 59.6 61.0 62.6 63.2 64.5 61.1
Supervised 77.0 76.7 76.5 75.7 75.4 77.1
Table 1. Top-1 accuracy of unsupervised ResNet-50 using linear
evaluation and supervised ResNet-505
, under varied color distor-
tion strength (see Appendix A) and other data transformations.
Strength 1 (+Blur) is our default data augmentation policy.
ric data augmentation hurts the performance. Nonetheless,
this setup should not substantively change the impact of
individual data augmentations or their compositions.
Figure 5 shows linear evaluation results under individual
and composition of transformations. We observe that no
single transformation suffices to learn good representations,
even though the model can almost perfectly identify the
positive pairs in the contrastive task. When composing aug-
mentations, the contrastive prediction task becomes harder,
but the quality of representation improves dramatically. Ap-
pendix B.2 provides a further study on composing broader
set of augmentations.
One composition of augmentations stands out: random crop-
ping and random color distortion. We conjecture that one
serious issue when using only random cropping as data
augmentation is that most patches from an image share a
similar color distribution. Figure 6 shows that color his-
tograms alone suffice to distinguish images. Neural nets
may exploit this shortcut to solve the predictive task. There-
fore, it is critical to compose cropping with color distortion
in order to learn generalizable features.
3.2. Contrastive learning needs stronger data
augmentation than supervised learning
To further demonstrate the importance of the color aug-
mentation, we adjust the strength of color augmentation as
5
Supervised models are trained for 90 epochs; longer training
improves performance of stronger augmentation by ∼ 0.5%.
0 50 100 150 200 250 300 350 400 450
Number of Parameters (Millions)
50
55
60
65
70
75
80
Top
1
R101
R101(2x)
R152
R152(2x)
R18
R18(2x)
R18(4x)
R34
R34(2x)
R34(4x)
R50
R50(2x)
R50(4x)
Sup. R50
Sup. R50(2x)
Sup. R50(4x)
R50*
R50(2x)*
R50(4x)*
Figure 7. Linear evaluation of models with varied depth and width.
Models in blue dots are ours trained for 100 epochs, models in red
stars are ours trained for 1000 epochs, and models in green crosses
are supervised ResNets trained for 90 epochs7
(He et al., 2016).
shown in Table 1. Stronger color augmentation substan-
tially improves the linear evaluation of the learned unsuper-
vised models. In this context, AutoAugment (Cubuk et al.,
2019), a sophisticated augmentation policy found using su-
pervised learning, does not work better than simple cropping
+ (stronger) color distortion. When training supervised mod-
els with the same set of augmentations, we observe that
stronger color augmentation does not improve or even hurts
their performance. Thus, our experiments show that unsu-
pervised contrastive learning benefits from stronger (color)
data augmentation than supervised learning. Although pre-
vious work has reported that data augmentation is useful
for self-supervised learning (Doersch et al., 2015; Bachman
et al., 2019; Hénaff et al., 2019; Asano et al., 2019), we
show that data augmentation that does not yield accuracy
benefits for supervised learning can still help considerably
with contrastive learning.
4. Architectures for Encoder and Head
4.1. Unsupervised contrastive learning benefits (more)
from bigger models
Figure 7 shows, perhaps unsurprisingly, that increasing
depth and width both improve performance. While similar
findings hold for supervised learning (He et al., 2016), we
find the gap between supervised models and linear classifiers
trained on unsupervised models shrinks as the model size
increases, suggesting that unsupervised learning benefits
more from bigger models than its supervised counterpart.
7
Training longer does not improve supervised ResNets (see
Appendix B.3).

Name Negative loss function Gradient w.r.t. u
NT-Xent uT
v+
/τ − log
P
v∈{v+,v−} exp(uT
v/τ) (1 − exp(uT
v+
/τ)
Z(u)
)/τv+
−
P
v−
exp(uT
v−
/τ)
Z(u)
/τv−
NT-Logistic log σ(uT
v+
/τ) + log σ(−uT
v−
/τ) (σ(−uT
v+
/τ))/τv+
− σ(uT
v−
/τ)/τv−
Margin Triplet − max(uT
v−
− uT
v+
+ m, 0) v+
− v−
if uT
v+
− uT
v−
< m else 0
Table 2. Negative loss functions and their gradients. All input vectors, i.e. u, v+
, v−
, are `2 normalized. NT-Xent is an abbreviation for
“Normalized Temperature-scaled Cross Entropy”. Different loss functions impose different weightings of positive and negative examples.
32 64
128
256
512
1024
2048
Projection output dimensionality
30
40
50
60
70
Top
1
Projection
Linear
Non-linear
None
Figure 8. Linear evaluation of representations with different pro-
jection heads g(·) and various dimensions of z = g(h). The
representation h (before projection) is 2048-dimensional here.
4.2. A nonlinear projection head improves the
representation quality of the layer before it
We then study the importance of including a projection
head, i.e. g(h). Figure 8 shows linear evaluation results
using three different architecture for the head: (1) identity
mapping; (2) linear projection, as used by several previous
approaches (Wu et al., 2018); and (3) the default nonlinear
projection with one additional hidden layer (and ReLU acti-
vation), similar to Bachman et al. (2019). We observe that a
nonlinear projection is better than a linear projection (+3%),
and much better than no projection (>10%). When a pro-
jection head is used, similar results are observed regardless
of output dimension. Furthermore, even when nonlinear
projection is used, the layer before the projection head, h,
is still much better (>10%) than the layer after, z = g(h),
which shows that the hidden layer before the projection
head is a better representation than the layer after.
We conjecture that the importance of using the representa-
tion before the nonlinear projection is due to loss of informa-
tion induced by the contrastive loss. In particular, z = g(h)
is trained to be invariant to data transformation. Thus, g can
remove information that may be useful for the downstream
task, such as the color or orientation of objects. By leverag-
ing the nonlinear transformation g(·), more information can
be formed and maintained in h. To verify this hypothesis,
we conduct experiments that use either h or g(h) to learn
to predict the transformation applied during the pretraining.
Here we set g(h) = W(2)
σ(W(1)
h), with the same input
and output dimensionality (i.e. 2048). Table 3 shows h
contains much more information about the transformation
applied, while g(h) loses information. Further analysis can
What to predict? Random guess
Representation
h g(h)
Color vs grayscale 80 99.3 97.4
Rotation 25 67.6 25.6
Orig. vs corrupted 50 99.5 59.6
Orig. vs Sobel filtered 50 96.6 56.3
Table 3. Accuracy of training additional MLPs on different repre-
sentations to predict the transformation applied. Other than crop
and color augmentation, we additionally and independently add
rotation (one of {0◦
, 90◦
, 180◦
, 270◦
}), Gaussian noise, and So-
bel filtering transformation during the pretraining for the last three
rows. Both h and g(h) are of the same dimensionality, i.e. 2048.
be found in Appendix B.4.
5. Loss Functions and Batch Size
5.1. Normalized cross entropy loss with adjustable
temperature works better than alternatives
We compare the NT-Xent loss against other commonly used
contrastive loss functions, such as logistic loss (Mikolov
et al., 2013), and margin loss (Schroff et al., 2015). Table
2 shows the objective function as well as the gradient to
the input of the loss function. Looking at the gradient, we
observe 1) `2 normalization (i.e. cosine similarity) along
with temperature effectively weights different examples, and
an appropriate temperature can help the model learn from
hard negatives; and 2) unlike cross-entropy, other objec-
tive functions do not weigh the negatives by their relative
hardness. As a result, one must apply semi-hard negative
mining (Schroff et al., 2015) for these loss functions: in-
stead of computing the gradient over all loss terms, one can
compute the gradient using semi-hard negative terms (i.e.,
those that are within the loss margin and closest in distance,
but farther than positive examples).
To make the comparisons fair, we use the same `2 normaliza-
tion for all loss functions, and we tune the hyperparameters,
and report their best results.8
Table 4 shows that, while
(semi-hard) negative mining helps, the best result is still
much worse than our default NT-Xent loss.
8
Details can be found in Appendix B.10. For simplicity, we
only consider the negatives from one augmentation view.

Margin NT-Logi. Margin (sh) NT-Logi.(sh) NT-Xent
50.9 51.6 57.5 57.9 63.9
Table 4. Linear evaluation (top-1) for models trained with different
loss functions. “sh” means using semi-hard negative mining.
`2 norm? τ Entropy Contrastive acc. Top 1
Yes
0.05 1.0 90.5 59.7
0.1 4.5 87.8 64.4
0.5 8.2 68.2 60.7
1 8.3 59.1 58.0
No
10 0.5 91.7 57.2
100 0.5 92.1 57.0
Table 5. Linear evaluation for models trained with different choices
of `2 norm and temperature τ for NT-Xent loss. The contrastive
distribution is over 4096 examples.
100 200 300 400 500 600 700 800 900 1000
Training epochs
50.0
52.5
55.0
57.5
60.0
62.5
65.0
67.5
70.0
Top
1
Batch size
256
512
1024
2048
4096
8192
Figure 9. Linear evaluation models (ResNet-50) trained with differ-
ent batch size and epochs. Each bar is a single run from scratch.10
We next test the importance of the `2 normalization (i.e.
cosine similarity vs dot product) and temperature τ in our
default NT-Xent loss. Table 5 shows that without normal-
ization and proper temperature scaling, performance is sig-
nificantly worse. Without `2 normalization, the contrastive
task accuracy is higher, but the resulting representation is
worse under linear evaluation.
5.2. Contrastive learning benefits (more) from larger
batch sizes and longer training
Figure 9 shows the impact of batch size when models are
trained for different numbers of epochs. We find that, when
the number of training epochs is small (e.g. 100 epochs),
larger batch sizes have a significant advantage over the
smaller ones. With more training steps/epochs, the gaps
between different batch sizes decrease or disappear, pro-
vided the batches are randomly resampled. In contrast to
10
A linear learning rate scaling is used here. Figure B.1 shows
using a square root learning rate scaling can improve performance
of ones with small batch sizes.
Method Architecture Param (M) Top 1 Top 5
Methods using ResNet-50:
Local Agg. ResNet-50 24 60.2 -
MoCo ResNet-50 24 60.6 -
PIRL ResNet-50 24 63.6 -
CPC v2 ResNet-50 24 63.8 85.3
SimCLR (ours) ResNet-50 24 69.3 89.0
Methods using other architectures:
Rotation RevNet-50 (4×) 86 55.4 -
BigBiGAN RevNet-50 (4×) 86 61.3 81.9
AMDIM Custom-ResNet 626 68.1 -
CMC ResNet-50 (2×) 188 68.4 88.2
MoCo ResNet-50 (4×) 375 68.6 -
CPC v2 ResNet-161 (∗) 305 71.5 90.1
SimCLR (ours) ResNet-50 (2×) 94 74.2 92.0
SimCLR (ours) ResNet-50 (4×) 375 76.5 93.2
Table 6. ImageNet accuracies of linear classifiers trained on repre-
sentations learned with different self-supervised methods.
Method Architecture
Label fraction
1% 10%
Top 5
Supervised baseline ResNet-50 48.4 80.4
Methods using other label-propagation:
Pseudo-label ResNet-50 51.6 82.4
VAT+Entropy Min. ResNet-50 47.0 83.4
UDA (w. RandAug) ResNet-50 - 88.5
FixMatch (w. RandAug) ResNet-50 - 89.1
S4L (Rot+VAT+En. M.) ResNet-50 (4×) - 91.2
Methods using representation learning only:
InstDisc ResNet-50 39.2 77.4
BigBiGAN RevNet-50 (4×) 55.2 78.8
PIRL ResNet-50 57.2 83.8
CPC v2 ResNet-161(∗) 77.9 91.2
SimCLR (ours) ResNet-50 75.5 87.8
SimCLR (ours) ResNet-50 (2×) 83.0 91.2
Table 7. ImageNet accuracy of models trained with few labels.
supervised learning (Goyal et al., 2017), in contrastive learn-
ing, larger batch sizes provide more negative examples,
facilitating convergence (i.e. taking fewer epochs and steps
for a given accuracy). Training longer also provides more
negative examples, improving the results. In Appendix B.1,
results with even longer training steps are provided.
6. Comparison with State-of-the-art
In this subsection, similar to Kolesnikov et al. (2019); He
et al. (2019), we use ResNet-50 in 3 different hidden layer
widths (width multipliers of 1×, 2×, and 4×). For better
convergence, our models here are trained for 1000 epochs.
Linear evaluation. Table 6 compares our results with previ-
ous approaches (Zhuang et al., 2019; He et al., 2019; Misra
& van der Maaten, 2019; Hénaff et al., 2019; Kolesnikov
et al., 2019; Donahue & Simonyan, 2019; Bachman et al.,

Food CIFAR10 CIFAR100 Birdsnap SUN397 Cars Aircraft VOC2007 DTD Pets Caltech-101 Flowers
Linear evaluation:
SimCLR (ours) 76.9 95.3 80.2 48.4 65.9 60.0 61.2 84.2 78.9 89.2 93.9 95.0
Supervised 75.2 95.7 81.2 56.4 64.9 68.8 63.8 83.8 78.7 92.3 94.1 94.2
Fine-tuned:
SimCLR (ours) 89.4 98.6 89.0 78.2 68.1 92.1 87.0 86.6 77.8 92.1 94.1 97.6
Supervised 88.7 98.3 88.7 77.8 67.0 91.4 88.0 86.5 78.8 93.2 94.2 98.0
Random init 88.3 96.0 81.9 77.0 53.7 91.3 84.8 69.4 64.1 82.7 72.5 92.5
Table 8. Comparison of transfer learning performance of our self-supervised approach with supervised baselines across 12 natural image
classification datasets, for ResNet-50 (4×) models pretrained on ImageNet. Results not significantly worse than the best (p > 0.05,
permutation test) are shown in bold. See Appendix B.8 for experimental details and results with standard ResNet-50.
2019; Tian et al., 2019) in the linear evaluation setting (see
Appendix B.6). Table 1 shows more numerical compar-
isons among different methods. We are able to use standard
networks to obtain substantially better results compared to
previous methods that require specifically designed archi-
tectures. The best result obtained with our ResNet-50 (4×)
can match the supervised pretrained ResNet-50.
Semi-supervised learning. We follow Zhai et al. (2019)
and sample 1% or 10% of the labeled ILSVRC-12 training
datasets in a class-balanced way (∼12.8 and ∼128 images
per class respectively). 11
We simply fine-tune the whole
base network on the labeled data without regularization
(see Appendix B.5). Table 7 shows the comparisons of
our results against recent methods (Zhai et al., 2019; Xie
et al., 2019; Sohn et al., 2020; Wu et al., 2018; Donahue &
Simonyan, 2019; Misra & van der Maaten, 2019; Hénaff
et al., 2019). The supervised baseline from (Zhai et al.,
2019) is strong due to intensive search of hyper-parameters
(including augmentation). Again, our approach significantly
improves over state-of-the-art with both 1% and 10% of the
labels. Interestingly, fine-tuning our pretrained ResNet-50
(2×, 4×) on full ImageNet are also significantly better then
training from scratch (up to 2%, see Appendix B.2).
Transfer learning. We evaluate transfer learning perfor-
mance across 12 natural image datasets in both linear evalu-
ation (fixed feature extractor) and fine-tuning settings. Fol-
lowing Kornblith et al. (2019), we perform hyperparameter
tuning for each model-dataset combination and select the
best hyperparameters on a validation set. Table 8 shows
results with the ResNet-50 (4×) model. When fine-tuned,
our self-supervised model significantly outperforms the su-
pervised baseline on 5 datasets, whereas the supervised
baseline is superior on only 2 (i.e. Pets and Flowers). On
the remaining 5 datasets, the models are statistically tied.
Full experimental details as well as results with the standard
ResNet-50 architecture are provided in Appendix B.8.
11
The details of sampling and exact subsets can be found in
https://www.tensorflow.org/datasets/catalog/imagenet2012_subset.
7. Related Work
The idea of making representations of an image agree with
each other under small transformations dates back to Becker
& Hinton (1992). We extend it by leveraging recent ad-
vances in data augmentation, network architecture and con-
trastive loss. A similar consistency idea, but for class label
prediction, has been explored in other contexts such as semi-
supervised learning (Xie et al., 2019; Berthelot et al., 2019).
Handcrafted pretext tasks. The recent renaissance of self-
supervised learning began with artificially designed pretext
tasks, such as relative patch prediction (Doersch et al., 2015),
solving jigsaw puzzles (Noroozi & Favaro, 2016), coloriza-
tion (Zhang et al., 2016) and rotation prediction (Gidaris
et al., 2018; Chen et al., 2019). Although good results
can be obtained with bigger networks and longer train-
ing (Kolesnikov et al., 2019), these pretext tasks rely on
somewhat ad-hoc heuristics, which limits the generality of
learned representations.
Contrastive visual representation learning. Dating back
to Hadsell et al. (2006), these approaches learn represen-
tations by contrasting positive pairs against negative pairs.
Along these lines, Dosovitskiy et al. (2014) proposes to
treat each instance as a class represented by a feature vector
(in a parametric form). Wu et al. (2018) proposes to use
a memory bank to store the instance class representation
vector, an approach adopted and extended in several recent
papers (Zhuang et al., 2019; Tian et al., 2019; He et al.,
2019; Misra & van der Maaten, 2019). Other work explores
the use of in-batch samples for negative sampling instead
of a memory bank (Doersch & Zisserman, 2017; Ye et al.,
2019; Ji et al., 2019).
Recent literature has attempted to relate the success of their
methods to maximization of mutual information between
latent representations (Oord et al., 2018; Hénaff et al., 2019;
Hjelm et al., 2018; Bachman et al., 2019). However, it is not
clear if the success of contrastive approaches is determined
by the mutual information, or by the specific form of the
contrastive loss (Tschannen et al., 2019).

We note that almost all individual components of our frame-
work have appeared in previous work, although the specific
instantiations may be different. The superiority of our frame-
work relative to previous work is not explained by any single
design choice, but by their composition. We provide a com-
prehensive comparison of our design choices with those of
previous work in Appendix C.
8. Conclusion
In this work, we present a simple framework and its in-
stantiation for contrastive visual representation learning.
We carefully study its components, and show the effects
of different design choices. By combining our findings,
we improve considerably over previous methods for self-
supervised, semi-supervised, and transfer learning.
Our approach differs from standard supervised learning on
ImageNet only in the choice of data augmentation, the use of
a nonlinear head at the end of the network, and the loss func-
tion. The strength of this simple framework suggests that,
despite a recent surge in interest, self-supervised learning
remains undervalued.
Acknowledgements
We would like to thank Xiaohua Zhai, Rafael Müller and
Yani Ioannou for their feedback on the draft. We are also
grateful for general support from Google Research teams in
Toronto and elsewhere.
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A. Data Augmentation Details
In our default pretraining setting (which is used to train our best models), we utilize random crop (with resize and random
flip), random color distortion, and random Gaussian blur as the data augmentations. The details of these three augmentations
are provided below.
Random crop and resize to 224x224 We use standard Inception-style random cropping (Szegedy et al., 2015). The
crop of random size (uniform from 0.08 to 1.0 in area) of the original size and a random aspect ratio (default: of
3/4 to 4/3) of the original aspect ratio is made. This crop is finally resized to the original size. This has been imple-
mented in Tensorflow as “slim.preprocessing.inception_preprocessing.distorted_bounding_box_crop”, or in Pytorch
as “torchvision.transforms.RandomResizedCrop”. Additionally, the random crop (with resize) is always followed by a
random horizontal/left-to-right flip with 50% probability. This is helpful but not essential. By removing this from our default
augmentation policy, the top-1 linear evaluation drops from 64.5% to 63.4% for our ResNet-50 model trained in 100 epochs.
Color distortion Color distortion is composed by color jittering and color dropping. We find stronger color jittering
usually helps, so we set a strength parameter.
A pseudo-code for color distortion using TensorFlow is as follows.
import tensorflow as tf
def color_distortion(image, s=1.0):
# image is a tensor with value range in [0, 1].
# s is the strength of color distortion.
def color_jitter(x):
# one can also shuffle the order of following augmentations
# each time they are applied.
x = tf.image.random_brightness(x, max_delta=0.8*s)
x = tf.image.random_contrast(x, lower=1-0.8*s, upper=1+0.8*s)
x = tf.image.random_saturation(x, lower=1-0.8*s, upper=1+0.8*s)
x = tf.image.random_hue(x, max_delta=0.2*s)
x = tf.clip_by_value(x, 0, 1)
return x
def color_drop(x):
image = tf.image.rgb_to_grayscale(image)
image = tf.tile(image, [1, 1, 3])
# randomly apply transformation with probability p.
image = random_apply(color_jitter, image, p=0.8)
image = random_apply(color_drop, image, p=0.2)
return image
A pseudo-code for color distortion using Pytorch is as follows 12
.
from torchvision import transforms
def get_color_distortion(s=1.0):
# s is the strength of color distortion.
color_jitter = transforms.ColorJitter(0.8*s, 0.8*s, 0.8*s, 0.2*s)
rnd_color_jitter = transforms.RandomApply([color_jitter], p=0.8)
rnd_gray = transforms.RandomGrayscale(p=0.2)
color_distort = transforms.Compose([
rnd_color_jitter,
rnd_gray])
12
Our code and results are based on Tensorflow, the Pytorch code here is a reference.

return color_distort
Gaussian blur This augmentation is in our default policy. We find it helpful, as it improves our ResNet-50 trained for
100 epochs from 63.2% to 64.5%. We blur the image 50% of the time using a Gaussian kernel. We randomly sample
σ ∈ [0.1, 2.0], and the kernel size is set to be 10% of the image height/width.
B. Additional Experimental Results
B.1. Batch Size and Training Steps
Figure B.1 shows the top-5 accuracy on linear evaluation when trained with different batch sizes and training epochs. The
conclusion is very similar to top-1 accuracy shown before, except that the differences between different batch sizes and
training steps seems slightly smaller here.
In both Figure 9 and Figure B.1, we use a linear scaling of learning rate similar to (Goyal et al., 2017) when training
with different batch sizes. Although linear learning rate scaling is popular with SGD/Momentum optimizer, we find a
square root learning rate scaling is more desirable with LARS optimizer. With square root learning rate scaling, we have
LearningRate = 0.075 ×
√
BatchSize, instead of LearningRate = 0.3 × BatchSize/256 in the linear scaling case, but
the learning rate is the same under both scaling methods when batch size of 4096 (our default batch size). A comparison is
presented in Table B.1, where we observe that square root learning rate scaling improves the performance for models trained
with small batch sizes and in smaller number of epochs.
Batch size Epochs 100 200 400 800
256 57.5 / 62.8 61.9 / 64.3 64.7 / 65.7 66.6 / 66.5
512 60.7 / 63.8 64.0 / 65.6 66.2 / 66.7 67.8 / 67.4
1024 62.8 / 64.3 65.3 / 66.1 67.2 / 67.2 68.5 / 68.3
2048 64.0 / 64.7 66.1 / 66.8 68.1 / 67.9 68.9 / 68.8
4096 64.6 / 64.5 66.5 / 66.8 68.2 / 68.0 68.9 / 69.1
8192 64.8 / 64.8 66.6 / 67.0 67.8 / 68.3 69.0 / 69.1
Table B.1. Linear evaluation (top-1) under different batch sizes and training epochs. On the left side of slash sign are models trained with
linear LR scaling, and on the right are models trained with square root LR scaling. The result is bolded if it is more than 0.5% better.
Square root LR scaling works better for smaller batch size trained in fewer epochs (with LARS optimizer).
We also train with larger batch size (up to 32K) and longer (up to 3200 epochs), with the square root learning rate scaling. A
shown in Figure B.2, the performance seems to saturate with a batch size of 8192, while training longer can still significantly
improve the performance.
100 200 300 400 500 600 700 800 900 1000
Training epochs
70.0
72.5
75.0
77.5
80.0
82.5
85.0
87.5
90.0
Top
5
Batch size
256
512
1024
2048
4096
8192
Figure B.1. Linear evaluation (top-5) of ResNet-50 trained with
different batch sizes and epochs. Each bar is a single run from
scratch. See Figure 9 for top-1 accuracy.
50 100 200 400 800 1600 3200
Training epochs
60
62
64
66
68
70
72
Top
1
Batch size
256
512
1024
2048
4096
8192
16384
32768
Figure B.2. Linear evaluation (top-1) of ResNet-50 trained with
different batch sizes and longer epochs. Here a square root learn-
ing rate, instead of a linear one, is utilized.

B.2. Broader composition of data augmentations further improves performance
Our best results in the main text (Table 6 and 7) can be further improved when expanding the default augmentation policy to
include the following: (1) Sobel filtering, (2) additional color distortion (equalize, solarize), and (3) motion blur. For linear
evaluation protocol, the ResNet-50 models (1×, 2×, 4×) trained with broader data augmentations achieve 70.0 (+0.7), 74.4
(+0.2), 76.8 (+0.3), respectively.
Table B.2 shows ImageNet accuracy obtained by fine-tuning the SimCLR model (see Appendix B.5 for the details of
fine-tuning procedure). Interestingly, when fine-tuned on full (100%) ImageNet training set, our ResNet (4×) model
achieves 80.4% top-1 / 95.4% top-5 13
, which is significantly better than that (78.4% top-1 / 94.2% top-5) of training from
scratch using the same set of augmentations (i.e. random crop and horizontal flip). For ResNet-50 (2×), fine-tuning our
pre-trained ResNet-50 (2×) is also better than training from scratch (77.8% top-1 / 93.9% top-5). There is no improvement
from fine-tuning for ResNet-50.
Architecture
Label fraction
1% 10% 100%
Top 1 Top 5 Top 1 Top 5 Top 1 Top 5
ResNet-50 49.4 76.6 66.1 88.1 76.0 93.1
ResNet-50 (2×) 59.4 83.7 71.8 91.2 79.1 94.8
ResNet-50 (4×) 64.1 86.6 74.8 92.8 80.4 95.4
Table B.2. Classification accuracy obtained by fine-tuning the SimCLR (which is pretrained with broader data augmentations) on 1%,
10% and full of ImageNet. As a reference, our ResNet-50 (4×) trained from scratch on 100% labels achieves 78.4% top-1 / 94.2% top-5.
B.3. Effects of Longer Training for Supervised Models
Here we perform experiments to see how training steps and stronger data augmentation affect supervised training. We test
ResNet-50 and ResNet-50 (4×) under the same set of data augmentations (random crops, color distortion, 50% Gaussian
blur) as used in our unsupervised models. Figure B.3 shows the top-1 accuracy. We observe that there is no significant
benefit from training supervised models longer on ImageNet. Stronger data augmentation slightly improves the accuracy of
ResNet-50 (4×) but does not help on ResNet-50. When stronger data augmentation is applied, ResNet-50 generally requires
longer training (e.g. 500 epochs 14
) to obtain the optimal result, while ResNet-50 (4×) does not benefit from longer training.
Model Training epochs
Top 1
Crop +Color +Color+Blur
ResNet-50
90 76.5 75.6 75.3
500 76.2 76.5 76.7
1000 75.8 75.2 76.4
ResNet-50 (4×)
90 78.4 78.9 78.7
500 78.3 78.4 78.5
1000 77.9 78.2 78.3
Table B.3. Top-1 accuracy of supervised models trained longer under various data augmentation procedures (from the same set of data
augmentations for contrastive learning).
B.4. Understanding The Non-Linear Projection Head
Figure B.3 shows the eigenvalue distribution of linear projection matrix W ∈ R2048×2048
used to compute z = Wh. This
matrix has relatively few large eigenvalues, indicating that it is approximately low-rank.
Figure B.4 shows t-SNE (Maaten & Hinton, 2008) visualizations of h and z = g(h) for randomly selected 10 classes by
our best ResNet-50 (top-1 linear evaluation 69.3%). Classes represented by h are better separated compared to z.
13
It is 80.1% top-1 / 95.2% top-5 without broader augmentations for pretraining SimCLR.
14
With AutoAugment (Cubuk et al., 2019), optimal test accuracy can be achieved between 900 and 500 epochs.

0 500 1000 1500 2000
Ranking
0
2
4
6
8
10
12
14
16
Squared
eigenvalue
(a) Y-axis in uniform scale.
0 500 1000 1500 2000
Ranking
10−11
10−9
10−7
10−5
10−3
10−1
101
Squared
eigenvalue
(b) Y-axis in log scale.
Figure B.3. Squared real eigenvalue distribution of linear projection
matrix W ∈ R2048×2048
used to compute g(h) = Wh.
(a) h (b) z = g(h)
Figure B.4. t-SNE visualizations of hidden vectors of images from
a randomly selected 10 classes in the validation set.
B.5. Semi-supervised Learning via Fine-Tuning
Fine-tuning Procedure We fine-tune using the Nesterov momentum optimizer with a batch size of 4096, momentum of
0.9, and a learning rate of 0.8 (following LearningRate = 0.05×BatchSize/256) without warmup. Only random cropping
(with random left-to-right flipping and resizing to 224x224) is used for preprocessing. We do not use any regularization
(including weight decay). For 1% labeled data we fine-tune for 60 epochs, and for 10% labeled data we fine-tune for 30
epochs. For the inference, we resize the given image to 256x256, and take a single center crop of 224x224.
Table B.4 shows the comparisons of top-1 accuracy for different methods for semi-supervised learning. Our models
significantly improve state-of-the-art.
Method Architecture
Label fraction
1% 10%
Top 1
Supervised baseline ResNet-50 25.4 56.4
Methods using label-propagation:
UDA (w. RandAug) ResNet-50 - 68.8
FixMatch (w. RandAug) ResNet-50 - 71.5
S4L (Rot+VAT+Ent. Min.) ResNet-50 (4×) - 73.2
Methods using self-supervised representation learning only:
CPC v2 ResNet-161(∗) 52.7 73.1
SimCLR (ours) ResNet-50 48.3 65.6
Table B.4. ImageNet top-1 accuracy of models trained with few labels. See Table 7 for top-5 accuracy.
B.6. Linear Evaluation
For linear evaluation, we follow similar procedure as fine-tuning (described in Appendix B.5), except that a larger learning
rate of 1.6 (following LearningRate = 0.1 × BatchSize/256) and longer training of 90 epochs. Alternatively, using LARS
optimizer with the pretraining hyper-parameters also yield similar results. Furthermore, we find that attaching the linear
classifier on top of the base encoder (with a stop_gradient on the input to linear classifier to prevent the label information
from influencing the encoder) and train them simultaneously during the pretraining achieves similar performance.
B.7. Correlation Between Linear Evaluation and Fine-Tuning
Here we study the correlation between linear evaluation and fine-tuning under different settings of training step and network
architecture.
Figure B.5 shows linear evaluation versus fine-tuning when training epochs of a ResNet-50 (using batch size of 4096) are
varied from 50 to 3200 as in Figure B.2. While they are almost linearly correlated, it seems fine-tuning on a small fraction

of labels benefits more from training longer.
62 64 66 68 70
Linear eval
35.0
37.5
40.0
42.5
45.0
47.5
50.0
Fine-tuning
on
1%
62 64 66 68 70
Linear eval
60
62
64
66
Fine-tuning
on
10%
Figure B.5. Top-1 accuracy of models trained in different epochs (from Figure B.2), under linear evaluation and fine-tuning.
Figure B.6 shows shows linear evaluation versus fine-tuning for different architectures of choice.
50 55 60 65 70
Linear eval
30
33
36
39
42
45
48
51
54
Finetune
(1%)
Width
1x
2x
4x
Depth
18
34
50
101
152
50 55 60 65 70
Linear eval
51
54
57
60
63
66
69
Finetune
(10%)
Width
1x
2x
4x
Depth
18
34
50
101
152
Figure B.6. Top-1 accuracy of different architectures under linear evaluation and fine-tuning.
B.8. Transfer Learning
We evaluated the performance of our self-supervised representation for transfer learning in two settings: linear evaluation,
where a logistic regression classifier is trained to classify a new dataset based on the self-supervised representation learned
on ImageNet, and fine-tuning, where we allow all weights to vary during training. In both cases, we follow the approach
described by Kornblith et al. (2019), although our preprocessing differs slightly.
B.8.1. METHODS
Datasets We investigated transfer learning performance on the Food-101 dataset (Bossard et al., 2014), CIFAR-10
and CIFAR-100 (Krizhevsky & Hinton, 2009), Birdsnap (Berg et al., 2014), the SUN397 scene dataset (Xiao et al.,
2010), Stanford Cars (Krause et al., 2013), FGVC Aircraft (Maji et al., 2013), the PASCAL VOC 2007 classification
task (Everingham et al., 2010), the Describable Textures Dataset (DTD) (Cimpoi et al., 2014), Oxford-IIIT Pets (Parkhi et al.,
2012), Caltech-101 (Fei-Fei et al., 2004), and Oxford 102 Flowers (Nilsback & Zisserman, 2008). We follow the evaluation
protocols in the papers introducing these datasets, i.e., we report top-1 accuracy for Food-101, CIFAR-10, CIFAR-100,
Birdsnap, SUN397, Stanford Cars, and DTD; mean per-class accuracy for FGVC Aircraft, Oxford-IIIT Pets, Caltech-101,
and Oxford 102 Flowers; and the 11-point mAP metric as defined in Everingham et al. (2010) for PASCAL VOC 2007. For
DTD and SUN397, the dataset creators defined multiple train/test splits; we report results only for the first split. Caltech-101
defines no train/test split, so we randomly chose 30 images per class and test on the remainder, for fair comparison with
previous work (Donahue et al., 2014; Simonyan & Zisserman, 2014).
We used the validation sets specified by the dataset creators to select hyperparameters for FGVC Aircraft, PASCAL VOC

2007, DTD, and Oxford 102 Flowers. For other datasets, we held out a subset of the training set for validation while
performing hyperparameter tuning. After selecting the optimal hyperparameters on the validation set, we retrained the
model using the selected parameters using all training and validation images. We report accuracy on the test set.
Transfer Learning via a Linear Classifier We trained an `2-regularized multinomial logistic regression classifier on
features extracted from the frozen pretrained network. We used L-BFGS to optimize the softmax cross-entropy objective
and we did not apply data augmentation. As preprocessing, all images were resized to 224 pixels along the shorter side
using bicubic resampling, after which we took a 224 × 224 center crop. We selected the `2 regularization parameter from a
range of 45 logarithmically spaced values between 10−6
and 105
.
Transfer Learning via Fine-Tuning We fine-tuned the entire network using the weights of the pretrained network as
initialization. We trained for 20,000 steps at a batch size of 256 using SGD with Nesterov momentum with a momentum
parameter of 0.9. We set the momentum parameter for the batch normalization statistics to max(1 − 10/s, 0.9) where s is
the number of steps per epoch. As data augmentation during fine-tuning, we performed only random crops with resize and
flips; in contrast to pretraining, we did not perform color augmentation or blurring. At test time, we resized images to 256
pixels along the shorter side and took a 224 × 224 center crop. (Additional accuracy improvements may be possible with
further optimization of data augmentation, particularly on the CIFAR-10 and CIFAR-100 datasets.) We selected the learning
rate and weight decay, with a grid of 7 logarithmically spaced learning rates between 0.0001 and 0.1 and 7 logarithmically
spaced values of weight decay between 10−6
and 10−3
, as well as no weight decay. We divide these values of weight decay
by the learning rate.
Training from Random Initialization We trained the network from random initialization using the same procedure
as for fine-tuning, but for longer, and with an altered hyperparameter grid. We chose hyperparameters from a grid of 7
logarithmically spaced learning rates between 0.001 and 1.0 and 8 logarithmically spaced values of weight decay between
10−5
and 10−1.5
. Importantly, our random initialization baselines are trained for 40,000 steps, which is sufficiently long to
achieve near-maximal accuracy, as demonstrated in Figure 8 of Kornblith et al. (2019).
On Birdsnap, there are no statistically significant differences among methods, and on Food-101, Stanford Cars, and FGVC
Aircraft datasets, fine-tuning provides only a small advantage over training from random initialization. However, on the
remaining 8 datasets, pretraining has clear advantages.
Supervised Baselines We compare against architecturally identical ResNet models trained on ImageNet with standard
cross-entropy loss. These models are trained with the same data augmentation as our self-supervised models (crops, strong
color augmentation, and blur) and are also trained for 1000 epochs. We found that, although stronger data augmentation and
longer training time do not benefit accuracy on ImageNet, these models performed significantly better than a supervised
baseline trained for 90 epochs and ordinary data augmentation for linear evaluation on a subset of transfer datasets. The
supervised ResNet-50 baseline achieves 76.3% top-1 accuracy on ImageNet, vs. 69.3% for the self-supervised counterpart,
while the ResNet-50 (4×) baseline achieves 78.3%, vs. 76.5% for the self-supervised model.
Statistical Significance Testing We test for the significance of differences between model with a permutation test. Given
predictions of two models, we generate 100,000 samples from the null distribution by randomly exchanging predictions
for each example and computing the difference in accuracy after performing this randomization. We then compute the
percentage of samples from the null distribution that are more extreme than the observed difference in predictions. For top-1
accuracy, this procedure yields the same result as the exact McNemar test. The assumption of exchangeability under the null
hypothesis is also valid for mean per-class accuracy, but not when computing average precision curves. Thus, we perform
significance testing for a difference in accuracy on VOC 2007 rather than a difference in mAP. A caveat of this procedure is
that it does not consider run-to-run variability when training the models, only variability arising from using a finite sample
of images for evaluation.
B.8.2. RESULTS WITH STANDARD RESNET
The ResNet-50 (4×) results shown in Table 8 of the text show no clear advantage to the supervised or self-supervised models.
With the narrower ResNet-50 architecture, however, supervised learning maintains a clear advantage over self-supervised
learning. The supervised ResNet-50 model outperforms the self-supervised model on all datasets with linear evaluation,
and most (10 of 12) datasets with fine-tuning. The weaker performance of the ResNet model compared to the ResNet (4×)

Food CIFAR10 CIFAR100 Birdsnap SUN397 Cars Aircraft VOC2007 DTD Pets Caltech-101 Flowers
Linear evaluation:
SimCLR (ours) 68.4 90.6 71.6 37.4 58.8 50.3 50.3 80.5 74.5 83.6 90.3 91.2
Supervised 72.3 93.6 78.3 53.7 61.9 66.7 61.0 82.8 74.9 91.5 94.5 94.7
Fine-tuned:
SimCLR (ours) 88.2 97.7 85.9 75.9 63.5 91.3 88.1 84.1 73.2 89.2 92.1 97.0
Supervised 88.3 97.5 86.4 75.8 64.3 92.1 86.0 85.0 74.6 92.1 93.3 97.6
Random init 86.9 95.9 80.2 76.1 53.6 91.4 85.9 67.3 64.8 81.5 72.6 92.0
Table B.5. Comparison of transfer learning performance of our self-supervised approach with supervised baselines across 12 natural
image datasets, using ImageNet-pretrained ResNet models. See also Figure 8 for results with the ResNet (4×) architecture.
model may relate to the accuracy gap between the supervised and self-supervised models on ImageNet. The self-supervised
ResNet gets 69.3% top-1 accuracy, 6.8% worse than the supervised model in absolute terms, whereas the self-supervised
ResNet (4×) model gets 76.5%, which is only 1.8% worse than the supervised model.
B.9. CIFAR-10
While we focus on using ImageNet as the main dataset for pretraining our unsupervised model, our method also works with
other datasets. We demonstrate it by testing on CIFAR-10 as follows.
Setup As our goal is not to optimize CIFAR-10 performance, but rather to provide further confirmation of our observations
on ImageNet, we use the same architecture (ResNet-50) for CIFAR-10 experiments. Because CIFAR-10 images are much
smaller than ImageNet images, we replace the first 7x7 Conv of stride 2 with 3x3 Conv of stride 1, and also remove the first
max pooling operation. For data augmentation, we use the same Inception crop (flip and resize to 32x32) as ImageNet,15
and
color distortion (strength=0.5), leaving out Gaussian blur. We pretrain with learning rate in {0.5, 1.0, 1.5}, temperature in
{0.1, 0.5, 1.0}, and batch size in {256, 512, 1024, 2048, 4096}. The rest of the settings (including optimizer, weight decay,
etc.) are the same as our ImageNet training.
Our best model trained with batch size 1024 can achieve a linear evaluation accuracy of 94.0%, compared to 95.1% from the
supervised baseline using the same architecture and batch size. The best self-supervised model that reports linear evaluation
result on CIFAR-10 is AMDIM (Bachman et al., 2019), which achieves 91.2% with a model 25× larger than ours. We note
that our model can be improved by incorporating extra data augmentations as well as using a more suitable base network.
Performance under different batch sizes and training steps Figure B.7 shows the linear evaluation performance under
different batch sizes and training steps. The results are consistent with our observations on ImageNet, although the largest
batch size of 4096 seems to cause a small degradation in performance on CIFAR-10.
100 200 300 400 500 600 700 800 900 1000
Training epochs
80
82
84
86
88
90
92
94
Top
1
Batch size
256
512
1024
2048
4096
Figure B.7. Linear evaluation of ResNet-50 (with ad-
justed stem) trained with different batch size and
epochs on CIFAR-10 dataset. Each bar is averaged
over 3 runs with different learning rates (0.5, 1.0,
1.5) and temperature τ = 0.5. Error bar denotes
standard deviation.
15
It is worth noting that, although CIFAR-10 images are much smaller than ImageNet images and image size does not differ among
examples, cropping with resizing is still a very effective augmentation for contrastive learning.

Optimal temperature under different batch sizes Figure B.8 shows the linear evaluation of model trained with three
different temperatures under various batch sizes. We find that when training to convergence (e.g. training epochs > 300), the
optimal temperature in {0.1, 0.5, 1.0} is 0.5 and seems consistent regardless of the batch sizes. However, the performance
with τ = 0.1 improves as batch size increases, which may suggest a small shift of optimal temperature towards 0.1.
256 512 1024 2048 4096
Batch size
75.0
77.5
80.0
82.5
85.0
87.5
90.0
92.5
95.0
Top
1
Temperature
0.1
0.5
1.0
(a) Training epochs ≤ 300
256 512 1024 2048 4096
Batch size
90
91
92
93
94
95
Top
1
Temperature
0.1
0.5
1.0
(b) Training epochs > 300
Figure B.8. Linear evaluation of the model (ResNet-50) trained with three temperatures on different batch sizes on CIFAR-10. Each bar is
averaged over multiple runs with different learning rates and total train epochs. Error bar denotes standard deviation.
B.10. Tuning For Other Loss Functions
The learning rate that works best for NT-Xent loss may not be a good learning rate for other loss functions. To ensure a
fair comparison, we also tune hyperparameters for both margin loss and logistic loss. Specifically, we tune learning rate
in {0.01, 0.1, 0.3, 0.5, 1.0} for both loss functions. We further tune the margin in {0, 0.4, 0.8, 1.6} for margin loss, the
temperature in {0.1, 0.2, 0.5, 1.0} for logistic loss. For simplicity, we only consider the negatives from one augmentation
view (instead of both sides), which slightly impairs performance but ensures fair comparison.
C. Further Comparison to Related Methods
As we have noted in the main text, most individual components of SimCLR have appeared in previous work, and the
improved performance is a result of a combination of these design choices. Table C.1 provides a high-level comparison of
the design choices of our method with those of previous methods. Compared with previous work, our design choices are
generally simpler.
Model Data Augmentation Base Encoder Projection Head Loss Batch Size Train Epochs
CPC v2 Custom ResNet-161 (modified) PixelCNN Xent 512#
∼200
AMDIM Fast AutoAug. Custom ResNet Non-linear MLP Xent w/ clip,reg 1008#
150
CMC Fast AutoAug. ResNet-50 (2×, L+ab) Linear layer Xent w/ `2, τ 156∗
280
MoCo Crop+color ResNet-50 (4×) Linear layer Xent w/ `2, τ 256∗
200
PIRL Crop+color ResNet-50 (2×) Linear layer Xent w/ `2, τ 1024∗
800
SimCLR Crop+color+blur ResNet-50 (4×) Non-linear MLP Xent w/ `2, τ 4096 1000
Table C.1. A high-level comparison of design choices and training setup (for best result on ImageNet) for each method. Note that
descriptions provided here are general; even when they match for two methods, formulations and implementations may differ (e.g. for
color augmentation). Refer to the original papers for more details. #
Examples are split into multiple patches, which enlarges the effective
batch size. ∗
A memory bank is employed.
In below, we provide an in-depth comparison of our method to the recently proposed contrastive representation learning
methods:
• DIM/AMDIM (Hjelm et al., 2018; Bachman et al., 2019) achieve global-to-local/local-to-neighbor prediction by
predicting the middle layer of ConvNet. The ConvNet is a ResNet that has bewen modified to place significant
constraints on the receptive fields of the network (e.g. replacing many 3x3 Convs with 1x1 Convs). In our framework,
we decouple the prediction task and encoder architecture, by random cropping (with resizing) and using the final

representations of two augmented views for prediction, so we can use standard and more powerful ResNets. Our
NT-Xent loss function leverages normalization and temperature to restrict the range of similarity scores, whereas they
use a tanh function with regularization. We use a simpler data augmentation policy, while they use FastAutoAugment
for their best result.
• CPC v1 and v2 (Oord et al., 2018; Hénaff et al., 2019) define the context prediction task using a deterministic strategy
to split examples into patches, and a context aggregation network (a PixelCNN) to aggregate these patches. The base
encoder network sees only patches, which are considerably smaller than the original image. We decouple the prediction
task and the encoder architecture, so we do not require a context aggregation network, and our encoder can look at the
images of wider spectrum of resolutions. In addition, we use the NT-Xent loss function, which leverages normalization
and temperature, whereas they use an unnormalized cross-entropy-based objective. We use simpler data augmentation.
• InstDisc, MoCo, PIRL (Wu et al., 2018; He et al., 2019; Misra & van der Maaten, 2019) generalize the Exemplar
approach originally proposed by Dosovitskiy et al. (2014) and leverage an explicit memory bank. We do not use a
memory bank; we find that, with a larger batch size, in-batch negative example sampling suffices. We also utilize a
nonlinear projection head, and use the representation before the projection head. Although we use similar types of
augmentations (e.g., random crop and color distortion), we expect specific parameters may be different.
• CMC (Tian et al., 2019) uses a separated network for each view, while we simply use a single network shared for all
randomly augmented views. The data augmentation, projection head and loss function are also different. We use larger
batch size instead of a memory bank.
• Whereas Ye et al. (2019) maximize similarity between augmented and unaugmented copies of the same image, we
apply data augmentation symmetrically to both branches of our framework (Figure 2). We also apply a nonlinear
projection on the output of base feature network, and use the representation before projection network, whereas Ye
et al. (2019) use the linearly projected final hidden vector as the representation. When training with large batch sizes
using multiple accelerators, we use global BN to avoid shortcuts that can greatly decrease representation quality.

A simple framework for contrastive learning of visual representations

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