"Walk me through how you'd calculate WACC" is one of the most frequently asked technical questions in finance interviews, and it rewards candidates who can lay out a clean, sequential process rather than jumping straight to the formula. Below is the four-step method interviewers expect, with a worked example you can follow line by line.
Step 1: Gather the Inputs
Before doing any math, you need seven inputs: the risk-free rate, the equity risk premium, the company's (levered) beta, its pre-tax cost of debt, its tax rate, and the market values of its equity and debt. In an interview, these are usually given to you or you're asked to make reasonable assumptions (e.g., the yield on a 10-year government bond for the risk-free rate).
Step 2: Calculate the Cost of Equity with CAPM
The Capital Asset Pricing Model gives the cost of equity: Re = Rf + (β × ERP), where Rf is the risk-free rate, β is the levered beta, and ERP is the equity risk premium. Using a risk-free rate of 4.0%, a beta of 1.2, and an equity risk premium of 5.5%: Re = 4.0% + (1.2 × 5.5%) = 4.0% + 6.6% = 10.6%. This is the annual return shareholders require for holding the stock, given its risk relative to the overall market.
Step 3: Calculate the After-Tax Cost of Debt
The cost of debt formula is: Rd (after-tax) = Rd × (1 − Tax Rate). With a pre-tax cost of debt of 6.0% and a tax rate of 25% (0.25): Rd (after-tax) = 6.0% × (1 − 0.25) = 6.0% × 0.75 = 4.5%. The tax adjustment matters because interest is tax-deductible — using the pre-tax rate here is one of the most common mistakes candidates make, and it overstates WACC.
Step 4: Weight by Market Value and Combine
Finally, weight each cost by its share of the company's total market-value capital structure: Weight of Equity = E / (D + E), Weight of Debt = D / (D + E). With a market value of equity of $800m and market value of debt of $200m: Weight of Equity = 800 / 1,000 = 80.0%, Weight of Debt = 200 / 1,000 = 20.0%. Then combine everything into the full WACC formula: WACC = (E/(D+E) × Re) + (D/(D+E) × Rd × (1 − T)). Plugging in the values calculated above: WACC = (0.80 × 10.6%) + (0.20 × 4.5%) = 8.48% + 0.90% = 9.38%.
Common Follow-Up Questions
Interviewers rarely stop at the number. Be ready to explain what happens to WACC as leverage increases (it initially falls, then rises again past a certain point as distress risk creeps into both the cost of debt and the cost of equity), why market values are used instead of book values for the weights, and how you'd estimate beta for a private company that has no observable share price.
Work Through the Full Case
For the complete version of this walkthrough — including the given data table, the formula reference for each step, and the full model answer — see WACC: The Building Blocks.