How do I calculate the after-tax cost of debt, and why does the tax shield matter for WACC?

The after-tax cost of debt represents the effective interest rate a company pays on its borrowings after accounting for the tax deductibility of interest expense. It is a key input to the Weighted Average Cost of Capital (WACC).

Formula:

r_d(after-tax) = YTM × (1 - t)

Where:

• YTM = yield to maturity on the company's existing debt (or new debt issuance)
• t = marginal corporate tax rate

Why YTM, Not the Coupon Rate?

The coupon rate is the contractual interest rate set at issuance. The YTM reflects the current market rate — what investors actually require to hold the bond today. Since WACC uses market-based costs, YTM is the correct input.

If a bond was issued at 5% coupon but now trades at a price where YTM = 6.5%, the company's current cost of debt is 6.5%, not 5%.

Why the Tax Shield?

Interest expense is tax-deductible for most companies. If a firm pays $1 million in interest at a 25$750,000 because the interest reduces taxable income, saving $250,000 in taxes. This tax shield lowers the effective cost of debt.

Worked Example — Pendleton Industries

Pendleton Industries has the following capital structure:

Component	Market Value	Weight
Equity	$600M	60%
Debt	$400M	40%

Debt details:

• Outstanding bonds: $1,000 par, 4.5% coupon, 8 years to maturity
• Current bond price: $962
• Marginal tax rate: 21%

Step 1 — Find YTM: Using a financial calculator or approximation:

• PV = -962, FV = 1000, PMT = 45, N = 8
• YTM ≈ 5.12%

Step 2 — Apply Tax Shield: r_d(after-tax) = 5.12% × (1 - 0.21) = 5.12% × 0.79 = 4.04%

Step 3 — Plug into WACC: If cost of equity = 11.3%:

WACC = (0.60 × 11.3%) + (0.40 × 4.04%) WACC = 6.78% + 1.62% = 8.40%

Common Mistakes:

1. Using the coupon rate instead of YTM — the coupon rate is historical, not current
2. Forgetting to apply the tax adjustment — always use after-tax cost in WACC
3. Using the book value of debt instead of market value for weights
4. Applying the tax shield to equity — only debt interest is tax-deductible (in most jurisdictions)

Exam Tip: If a question gives you a bond price and coupon, calculate YTM first, then apply the tax adjustment. If the question directly gives you the "cost of debt" or YTM, just multiply by (1 - t). Never forget the tax shield.

Explore more corporate finance problems in our CFA Level I materials.