WACC Deep Dive

WACC has two primary components: the cost of equity and the after-tax cost of debt. The cost of equity is typically derived using the Capital Asset Pricing Model (CAPM), which requires the risk-free rate, equity risk premium, and the company's beta. The cost of debt is the yield on the company's outstanding debt, tax-effected to reflect the interest tax shield. The weights are the market values of equity and debt as proportions of total enterprise capitalization — not book values. In practice, bankers use the company's target or optimal capital structure rather than the current capital structure if the company intends to change its leverage over time.

WACC contains a circularity problem: you need the market value of equity to calculate WACC, but you need WACC to discount cash flows and arrive at the market value of equity. In practice, this circularity is resolved through iteration. Start with an initial estimate of equity value (e.g., from comparable companies), compute WACC, discount the cash flows, derive a new equity value, recalculate WACC with the updated weight, and repeat until the equity value converges. Most Excel models handle this automatically with iterative calculations enabled (File > Options > Formulas > Enable iterative calculation). Alternatively, bankers sometimes sidestep the issue by using the target capital structure based on peer median leverage ratios, which avoids the circularity entirely and is the most common approach in practice.

WACC assumes a constant capital structure over the projection period because the tax shield is embedded in the discount rate itself. This assumption breaks down in leveraged buyouts and other situations where debt is paid down aggressively over time, causing the capital structure to shift dramatically. In these cases, the Adjusted Present Value (APV) method is more appropriate. APV discounts unlevered free cash flows at the unlevered cost of equity (no debt benefit) and then separately values the interest tax shields at either the cost of debt or unlevered cost of equity. APV is also preferred for highly distressed companies where the probability of realizing tax shields is uncertain. For stable companies with relatively constant leverage, WACC remains the standard approach in investment banking DCF models.

Small changes in WACC have an outsized impact on the terminal value, which typically represents 60-80% of total enterprise value in a DCF. For this reason, most models include a sensitivity analysis toggling WACC and the terminal growth rate. Common pitfalls include using book value weights instead of market value weights, forgetting to tax-effect the cost of debt, applying a single beta without considering the company's specific leverage (use unlevered beta and re-lever to the target structure), and using a WACC that is inconsistent with the cash flows being discounted — WACC should only be used to discount unlevered cash flows, not levered cash flows.