The cost of capital is the rate of return that the suppliers of capital—bondholders and owners—require as compensation for their contribution of capital. Another way of looking at the cost of capital is that it is the opportunity cost of funds for the suppliers of capital: A potential supplier of capital will not voluntarily invest in a company unless its return meets or exceeds what the supplier could earn elsewhere in an investment of comparable risk.

A company typically has several alternatives for raising capital, including issuing equity, debt, and instruments that share characteristics of debt and equity. Each source selected becomes a component of the company’s funding and has a cost (required rate of return) that may be called a component cost of capital. Because we are using the cost of capital in the evaluation of investment opportunities, we are dealing with a marginal cost—what it would cost to raise additional funds for the potential investment project. Therefore, the cost of capital that the investment analyst is concerned with is a marginal cost.

Let us focus on the cost of capital for the entire company (later we will address how to adjust that for specific projects). The cost of capital of a company is the required rate of return that investors demand for the average-risk investment of a company. The most common way to estimate this required rate of return is to calculate the marginal cost of each of the various sources of capital and then calculate a weighted average of these costs. This weighted average is referred to as the weighted average cost of capital (WACC). The WACC is also referred to as the marginal cost of capital (MCC) because it is the cost that a company incurs for additional capital. The weights in this weighted average are the proportions of the various sources of capital that the company uses to support its investment program. Therefore, the WACC, in its most general terms, is

Equation (1) 

WACC = w_dr_d(1 – t) + w_pr_p + w_er_e  

where

w_d = the proportion of debt that the company uses when it raises new funds

r_d = the before-tax marginal cost of debt

t = the company’s marginal tax rate

w_p = the proportion of preferred stock the company uses when it raises new funds

r_p = the marginal cost of preferred stock

w_e = the proportion of equity that the company uses when it raises new funds

r_e = the marginal cost of equity