WACC and CAPM Explained for Valuation Interviews

WACC (Weighted Average Cost of Capital) is the discount rate in a DCF — and the single most sensitive input in the entire model. A one-percentage-point change in WACC can move enterprise value by 10%–20% or more, especially for companies where most value sits in the terminal period. Interviewers test WACC constantly because it requires you to understand capital structure, market data, and the assumptions behind each component. This guide covers the full calculation and the judgment calls that separate a mechanical answer from a credible one.

What WACC represents

WACC is the blended average cost of all capital providers — debt and equity — weighted by their proportion of total capital:

WACC = (E/V × Cost of Equity) + (D/V × Cost of Debt × (1 − Tax Rate))

Where:

• E = market value of equity
• D = market value of debt
• V = E + D (total capital)
• Tax Rate = marginal corporate tax rate

WACC is the rate at which you discount unlevered free cash flow — cash available to all capital providers before any financing decisions. That's why UFCF pairs with WACC, and levered FCF pairs with cost of equity.

Component 1: Cost of Equity (CAPM)

The Capital Asset Pricing Model is the standard method:

Cost of Equity = Risk-Free Rate + Beta × Equity Risk Premium

Risk-Free Rate

Typically the yield on a 10-year U.S. Treasury bond — the closest proxy to a truly risk-free long-term rate. Use the current yield at the valuation date, not a historical average.

As of typical market conditions: 4.0%–4.5% range. Always state the rate you're using and the date/source.

Beta (β)

Beta measures the stock's sensitivity to overall market movements:

• β = 1.0: moves with the market
• β above 1.0: more volatile than the market (cyclical, high-growth)
• β below 1.0: less volatile (defensive, utilities, consumer staples)

Raw vs. adjusted beta:

Adjusted Beta = (2/3) × Raw Beta + (1/3) × 1.0

Adjusted beta pulls raw beta toward 1.0, reflecting the empirical observation that betas tend to revert to the mean over time. Most banks use adjusted beta in WACC calculations.

Unlevering and relevering beta (when using comps' betas for a private company or different capital structure):

Unlevered Beta = Levered Beta ÷ [1 + (1 − Tax Rate) × (Debt/Equity)]
Relevered Beta = Unlevered Beta × [1 + (1 − Tax Rate) × (Target Debt/Equity)]

This is tested in interviews: "How would you estimate beta for a private company?" Answer: unlever betas from comparable public companies, then relever to the target's capital structure.

Equity Risk Premium (ERP)

The extra return investors demand for holding equities vs. risk-free bonds. Common estimates:

• Historical ERP: ~5.0%–6.0% (based on long-run S&P 500 outperformance over Treasuries)
• Damodaran implied ERP: updated annually, typically 4.5%–5.5%
• Practitioner convention: often 5.0%–5.5% in models

State your assumption. Using 5.0% vs. 6.0% changes cost of equity by a full percentage point for a β = 1.0 stock.

Worked example: Cost of Equity

Risk-Free Rate:     4.25%
Adjusted Beta:      1.15
Equity Risk Premium: 5.00%
Cost of Equity = 4.25% + (1.15 × 5.00%) = 4.25% + 5.75% = 10.0%

Component 2: Cost of Debt

Cost of debt is the company's current market borrowing rate:

Cost of Debt = Yield to Maturity on existing bonds (or interest rate on recent debt issuance)

For companies without public bonds, approximate:

Cost of Debt ≈ Interest Expense ÷ Average Total Debt (rough proxy)

Or use the company's credit rating to estimate a spread over Treasuries:

• Investment grade (BBB/Baa): Rf + 150–250 bps
• High yield (BB/Ba): Rf + 300–500 bps

Tax shield: interest is tax-deductible, so the effective cost of debt is lower:

After-Tax Cost of Debt = Pre-Tax Cost of Debt × (1 − Tax Rate)

Example: 6.0% pre-tax cost × (1 − 25% tax rate) = 4.5% after-tax cost of debt.

Component 3: Capital structure weights

Use market values, not book values:

E/V = Market Cap ÷ (Market Cap + Market Value of Debt)
D/V = Market Value of Debt ÷ (Market Cap + Market Value of Debt)

Book value of equity routinely understates market equity for growing companies. Using book weights skews WACC toward whichever side looks cheaper on a book basis — usually debt — producing an artificially low discount rate.

Target vs. current weights: for stable companies, current market weights are fine. For companies actively changing capital structure (recent LBO, major debt paydown), use target weights if the change is material and permanent.

Full WACC calculation

Company Y:

• Market Cap: $3,000M
• Market Value of Debt: $1,000M
• Cost of Equity: 10.0%
• Pre-Tax Cost of Debt: 5.5%
• Tax Rate: 25%

E/V = $3,000M ÷ $4,000M = 75.0%
D/V = $1,000M ÷ $4,000M = 25.0%
After-Tax Cost of Debt = 5.5% × (1 − 0.25) = 4.125%

WACC = (75% × 10.0%) + (25% × 4.125%)
     = 7.50% + 1.03%
     = 8.53%

Why WACC is the most sensitive DCF input

WACC compounds through every discounted cash flow:

PV of Year 5 UFCF at 8.5% WACC: UFCF ÷ (1.085)^5
PV of Year 5 UFCF at 9.5% WACC: UFCF ÷ (1.095)^5

The difference grows with time — which matters enormously because terminal value (discounted from Year 5 or 10) often represents 60%–80% of total EV. A 1% WACC change can shift total EV by 10%–15% on a typical DCF.

This is why every real DCF includes a WACC sensitivity table:

| | Terminal Growth 2.0% | 2.5% | 3.0% | |--|----------------------|------|------| | WACC 7.5% | $42 | $45 | $49 | | WACC 8.5% | $35 | $38 | $41 | | WACC 9.5% | $29 | $32 | $34 |

Present the range, not a single point. A single WACC implies false precision.

Common interview questions

"Walk me through WACC." State the formula. Calculate cost of equity via CAPM (Rf, beta, ERP). Calculate after-tax cost of debt. Weight by market values of equity and debt. Show the math.

"How do you estimate beta for a private company?" Unlever betas from comparable public companies' levered betas, then relever to the target's capital structure using the Hamada equation.

"Why use market weights, not book weights?" Market values reflect what capital actually costs today. Book equity understates true equity value for most companies, skewing the weight toward debt and producing an artificially low WACC.

"What's the difference between WACC and cost of equity?" WACC is the blended rate for all capital providers — used to discount unlevered FCF. Cost of equity is the return equity investors require — used to discount levered FCF or in equity-only analyses. WACC is always lower than cost of equity because debt is cheaper (and tax-shielded).

"How does capital structure affect WACC?" Modigliani-Miller (with taxes): WACC initially declines as cheap debt replaces expensive equity, reaches a minimum at some optimal leverage, then rises as financial distress costs increase. In practice, WACC is relatively flat over a wide range of leverage for investment-grade companies.

Sanity checks

Before presenting WACC in a DCF:

| Check | Reasonable Range | |-------|-----------------| | Cost of Equity | 8%–12% for most companies | | After-Tax Cost of Debt | 3%–6% for investment grade | | WACC | 7%–11% for most operating companies | | Beta | 0.5–1.5 for most non-financial companies | | ERP | 4.5%–6.0% |

If your WACC is 6% or 14%, double-check every input. Extreme WACC values usually indicate a calculation error, not a genuinely unusual company.

The takeaway

WACC is not a single number — it's a range driven by judgment on beta, ERP, and capital structure weights. The interview skill is calculating it correctly, explaining each component, and presenting it as a sensitivity range rather than a false-precision point estimate. Master CAPM and the bridge from market data to discount rate, and the DCF becomes a defensible valuation instead of a spreadsheet exercise.