The weighted average cost of capital is not quite right

A firm's cost of capital used in discounted cash flow analysis is commonly calculated as a weighted average of the after tax costs of the firm's various sources of financing (equity, debt, preferred stock). Its use implies that for investment projects earning precisely the WACC the cash (in)flow is exactly sufficient to reward all the suppliers of finance with their respective costs of capital. However, the necessary cash flow (normal profit) implied by the WACC is inadequate to provide the cash flows to the individual sources of financing when they are considered separately. This note discusses the problem (WACC is a linear approximation of a nonlinear relationship) and presents a modification of the WACC which is conceptually superior to the WACC as commonly calculated.