At a Glance
- Always use market values (not book values) for equity and debt weights.
- Calculate cost of equity using CAPM: Re = Rf + β × (Rm − Rf).
- Apply the tax shield to the cost of debt: after-tax Rd = Rd × (1 − Tc).
- Weight each cost by its share of total capital (E/V and D/V).
- The result is the minimum return the company must earn to satisfy all capital providers.
Step 1 — Determine the Capital Structure
Use market values, not book values. For a public company, equity market cap is simply shares outstanding × current share price. For debt, use the market value of outstanding bonds and loans — book value is an acceptable approximation when market prices are unavailable.
If the company has preferred stock, it must be included as a third component with its own cost (the preferred dividend yield) and weight.
Step 2 — Calculate the Cost of Equity
The Capital Asset Pricing Model (CAPM) is the industry standard for estimating what equity investors require as a return. It compensates investors for the time value of money (risk-free rate) plus a premium for bearing market risk (beta × ERP).
| Input | Where to Find It |
|---|---|
| Risk-free rate (Rf) | Current 10-year US Treasury yield (FRED, Bloomberg) |
| Beta (β) | Yahoo Finance, Bloomberg, Capital IQ; use industry average for private firms |
| Expected market return (Rm) | Historical S&P 500 long-run average ≈ 10%; or use Rf + Damodaran ERP |
Step 3 — Calculate the After-Tax Cost of Debt
Interest payments are tax-deductible, which reduces the effective cost of debt. Use the company’s weighted average interest rate across all debt instruments, not the coupon rate on a single bond.
Use the marginal tax rate (the rate paid on the next dollar of income), not the effective rate from the income statement.
Step 4 — Apply the Weights and Calculate WACC
Worked Example — TechCo Inc.
| Input | Value |
|---|---|
| Equity Value (E) | $60,000,000 |
| Debt Value (D) | $40,000,000 |
| Risk-Free Rate (Rf) | 4.5% |
| Beta (β) | 1.2 |
| Expected Market Return (Rm) | 10.0% |
| Pre-Tax Cost of Debt (Rd) | 5.0% |
| Corporate Tax Rate (Tc) | 21% |
- Total firm value: V = $60M + $40M = $100M
- Equity weight = 60%; Debt weight = 40%
- Cost of equity (CAPM): Re = 4.5% + 1.2 × (10% − 4.5%) = 4.5% + 6.6% = 11.1%
- After-tax cost of debt: 5% × (1 − 21%) = 3.95%
- WACC = (60% × 11.1%) + (40% × 3.95%) = 6.66% + 1.58% = 8.24%
Interpretation: TechCo must earn at least 8.24% on every dollar of capital deployed to generate positive economic value. Any investment expected to return less than this rate destroys value for investors.
Common Mistakes
- Book value for equity — using balance sheet equity instead of market cap can understate or overstate the equity weight significantly.
- Coupon rate for cost of debt — old bonds may carry coupon rates far from today’s market yields; use the current yield-to-maturity.
- Wrong beta — using a peer’s levered beta without adjusting for your own capital structure introduces a systematic error.
- Effective tax rate instead of marginal — the effective rate smooths out prior-year tax benefits; the marginal rate is what matters for the next dollar of interest deduction.
- Ignoring preferred stock — if the company has preferred shares, they must be weighted at their own cost (preferred dividend / preferred price).
Related Concepts
- What is WACC?
- CAPM Formula: How to Calculate Cost of Equity
- WACC vs Hurdle Rate: What’s the Difference?
FAQ
Where do I get beta for a private company?
Find the median unlevered beta from a peer group of comparable public companies, then re-lever it for the private company’s capital structure using the Hamada equation: βL = βU × [1 + (1 − Tc) × (D/E)].
Should I use book value or market value of debt?
Market value is technically correct, but book value is a widely accepted approximation because debt prices don’t deviate from face value as dramatically as equity prices can.
How often should WACC be recalculated?
At minimum, annually. Also recalculate whenever capital structure changes materially (new debt issuance, equity raise, large share buyback) or when market interest rates shift by more than 50–100 basis points.
Can I use WACC to value a startup?
With caution. Startups typically have no debt and no observable beta, so CAPM inputs require significant judgment. A venture capital method or scenario-weighted DCF with a higher discount rate (20–35%) is often more practical.
DISCLAIMER: All calculations are illustrative only and do not constitute investment advice.