How to Calculate WACC in an Interview: The Formula, the Weights, and the Private Company Twist
"How do you calculate WACC?" sounds like a formula-recall question, and interviewers will let you answer it that way for about ten seconds before they start pulling on it. The real test is whether you can build each input from scratch, defend why you used market value instead of book value for the weights, and handle the version of the question that actually separates prepared candidates from memorizers: calculating WACC for a company that has no public stock price at all.
This comes up constantly as a stand-alone question, not just buried inside a DCF walkthrough, because WACC is the one input that can move an entire valuation by double digits with a small, defensible-sounding change. Here's the formula, where each piece actually comes from, and the private company version interviewers love to push on.
The WACC Formula
WACC blends the cost of equity and the after-tax cost of debt, weighted by how much of the company's capital structure each one represents:
WACC = (E/V) × Re + (D/V) × Rd × (1 − T)
E is the market value of equity, D is the market value of debt, V is E plus D (total capital), Re is the cost of equity, Rd is the pre-tax cost of debt, and T is the tax rate. The (1 − T) term exists because interest payments are tax-deductible, so the government effectively subsidizes part of the cost of debt. Equity gets no equivalent shield, which is one reason cost of equity almost always comes out higher than cost of debt.
Cost of Equity: Where CAPM Comes From
Interviewers expect the Capital Asset Pricing Model here, not an estimate pulled from thin air:
Re = Rf + Beta × ERP
Rf is the risk-free rate, typically the yield on a 10-year or 20-year Treasury bond. ERP is the equity risk premium, the extra return investors demand for holding stocks instead of risk-free government debt, usually sourced from published estimates like Damodaran's data. Beta measures how much the company's stock moves relative to the overall market: a beta of 1.3 means the stock tends to move 30% more than the market in either direction, so it demands a higher return to compensate for that extra volatility.
For a public company, you'd pull beta directly from a regression of its own stock returns against a market index. That's the easy case. The harder case, and the one worth preparing for specifically, is what happens when the company doesn't have a stock price to regress in the first place.
Cost of Debt and the Tax Shield
Cost of debt is usually easier to pin down than cost of equity. For a company with outstanding bonds or traded loans, use the yield to maturity on that debt. For a company without easily observable debt pricing, use the interest rate on comparable debt from similarly rated companies, or back into an implied rating using interest coverage ratios and a ratings-to-spread table.
Whichever source you use, apply the tax shield before it goes into WACC: after-tax cost of debt = Rd × (1 − T). Use the marginal tax rate the company actually pays, not an effective rate pulled off a messy income statement with one-time items in it. If an interviewer asks why the after-tax number matters, the answer is the one above: interest is deductible, so a dollar of debt costs the company less than a dollar of equity at the same stated rate.
The Weights: Market Value, Not Book Value
This is where candidates lose points even when the formula itself is memorized correctly. E and D need to be market values, not the numbers sitting on the balance sheet.
Market value of equity is just market capitalization: share price times diluted shares outstanding. Market value of debt is harder to get exactly right, since most corporate debt doesn't trade with the same visibility as equity, but the convention is to use the face value of debt as a reasonable proxy when a market price isn't available, since debt tends to trade closer to par than equity trades to book value.
The other detail interviewers check for: use the company's target capital structure, not its current one, if the two differ. A company that just took on debt for a one-time acquisition, or is paying down debt toward a long-term target, isn't going to carry today's leverage forever, and WACC is supposed to reflect the capital structure the company will actually operate under going forward.
A Worked Example: Calculating WACC for a Private Company
Here's the version of this question that trips up most candidates: calculating WACC for a company with no traded stock, which means no market cap and no beta to pull.
Say you're valuing Meridian Logistics, a private freight company. Since there's no stock price, you can't get Meridian's own beta directly, so you build it from public peers instead.
Step 1: Pull peer betas, debt-to-equity ratios, and tax rates.
| Peer | Levered Beta | D/E | Tax Rate |
|---|---|---|---|
| Peer A | 1.30 | 0.40 | 25% |
| Peer B | 1.50 | 0.60 | 24% |
| Peer C | 1.10 | 0.20 | 26% |
Step 2: Unlever each peer's beta to strip out the effect of its own capital structure, using Unlevered Beta = Levered Beta / [1 + (1 − T) × (D/E)]:
- Peer A: 1.30 / [1 + 0.75 × 0.40] = 1.00
- Peer B: 1.50 / [1 + 0.76 × 0.60] = 1.03
- Peer C: 1.10 / [1 + 0.74 × 0.20] = 0.96
Average unlevered beta: roughly 1.00. This is the beta the business itself carries, independent of how any one peer happens to be financed.
Step 3: Relever the average beta at Meridian's own target capital structure, say a D/E of 0.50 and a 25% tax rate, using Levered Beta = Unlevered Beta × [1 + (1 − T) × (D/E)]:
1.00 × [1 + 0.75 × 0.50] = 1.38
Step 4: Plug the relevered beta into CAPM. With a risk-free rate of 4.0% and an equity risk premium of 5.5%:
Re = 4.0% + 1.38 × 5.5% = 11.6%
Step 5: Estimate the cost of debt. Based on Meridian's credit profile and comparable loan pricing, assume lenders would price its debt at 7.5% pre-tax. After-tax cost of debt = 7.5% × (1 − 0.25) = 5.6%.
Step 6: Apply the weights. A target D/E of 0.50 implies debt is one-third of total capital and equity is two-thirds (D/(D+E) = 0.50 / 1.50 = 33.3%, E/(D+E) = 66.7%):
WACC = 0.667 × 11.6% + 0.333 × 5.6% = 7.7% + 1.9% = 9.6%
That's the full answer: a beta borrowed from the market, relevered onto the company's own balance sheet, blended with a credit-based cost of debt, weighted by a target capital structure instead of a book-value one.
Why Interviewers Push on the Private Company Case
The public-company version of WACC is mostly a lookup exercise: pull beta, pull market cap, pull a bond yield, plug it in. The private company version tests whether you actually understand why each input exists, because none of the shortcuts work. You can't look up a beta that doesn't exist. You can't weight by a market cap that was never set by trading. Every input has to be built or borrowed from somewhere else, and defended.
This is also why the unlevering and relevering step matters so much. A peer's levered beta reflects that peer's own debt load, and simply copying it onto a company with a different capital structure would overstate or understate the risk. Stripping out the peer's leverage, averaging the pure business risk across several peers, then re-adding leverage at the target's own structure is what makes the borrowed beta actually apply to the company you're valuing.
Common Follow-Ups and Mistakes
"Why not just use the risk-free rate on its own and skip beta entirely?" Because beta is what makes the discount rate specific to this company's risk. Two companies in the same industry with the same risk-free rate exposure can still have very different costs of equity if one carries far more operating or financial leverage than the other.
"What if the peer set has only one or two companies?" Flag it. A WACC built off a thin peer set is more fragile, and a strong candidate says so rather than presenting a one-peer average with false confidence. Widening the peer set, even to adjacent sub-industries, is usually the right move when options are thin.
"How much does WACC actually matter?" More than most inputs in a model. A change of even one percentage point in WACC can shift implied enterprise value by 10% to 15% in a typical DCF, since the discount rate compounds across every projected year and the terminal value. That sensitivity is exactly why interviewers spend so much time on how each input gets built rather than just the formula itself.
"Do you use the same tax rate for unlevering and relevering?" Not necessarily. Use each peer's own tax rate when stripping out its leverage, since that's the rate that applied when the leverage effect was created, and use the target company's own tax rate when relevering onto its capital structure.
WACC rewards the same kind of preparation as the DCF and enterprise value questions it feeds into: know the formula cold, but spend more time on where each input comes from and how it changes when the company you're valuing doesn't have a convenient stock price to pull numbers from.