Cost of Debt
The cost of debt is relatively simple to calculate, as it is composed of the rate of interest paid. In practice, the interest-rate paid by the company can be modeled as the risk-free rate plus a risk component (risk premium), which itself incorporates a probable rate of default (and amount of recovery given default). For companies with similar risk or credit ratings, the interest rate is largely exogenous (not linked to the cost of debt), the cost of equity is broadly defined as the risk-weighted projected return required by investors, where the return is largely unknown. The cost of equity is therefore inferred by comparing the investment to other investments (comparable) with similar risk profiles to determine the "market" cost of equity. It is commonly equated using the capital asset pricing model formula (below), although articles such as Stulz 1995 question the validity of using a local CAPM versus an international CAPM- also considering whether markets are fully integrated or segmented (if fully integrated, there would be no need for a local CAPM).
Once cost of debt and cost of equity have been determined, their blend, the weighted-average cost of capital (WACC), can be calculated. This WACC can then be used as a discount rate for a project's projected cash flows.
The effective rate that a company