Capital is comprised of debt, common equity, and preferred stock. The cost of capital is the average rate paid for using a firm's capital funds and provides a benchmark to evaluate investment returns. Investors provide capital by purchasing securities and receive returns adjusted for taxes and administrative expenses, which represent the firm's costs. Equity has the highest risk and cost while debt has the lowest risk and cost, with preferred stock in between. A firm's weighted average cost of capital is calculated using the costs of each capital component weighted by their dollar amounts in the firm's total capital structure.