Direct Answer
Cost of debt is the effective interest rate a company pays on its borrowings. It is commonly estimated from the current yield to maturity on the company's outstanding debt, or approximated as interest expense divided by average total debt. In a WACC calculation it is used on an after-tax basis, since interest expense is typically tax-deductible and that deduction reduces the effective cost of borrowing to the company.
Key Takeaways
- Cost of debt is the effective rate a company pays on its borrowings - a way of pricing what lenders and bondholders charge it to use their money.
- It is commonly estimated two ways: from the current yield to maturity on outstanding debt, or by dividing interest expense by average total debt.
- WACC uses the after-tax cost of debt, since interest expense is typically tax-deductible and lowers the effective cost to the company.
- Both estimation methods are simplifications - each has known limitations and neither is universally the "correct" answer.
- A rising cost of debt is one signal, among several, that the market perceives a company's credit risk as increasing.
What Is Cost of Debt?
Cost of debt describes what it actually costs a company to borrow - the effective interest rate paid across its loans, bonds, and other debt obligations. It is one of the two components, alongside cost of equity, that feed into the weighted average cost of capital (WACC), the discount rate commonly used in discounted cash flow and other valuation models.
Because a company typically carries several debt instruments issued at different times, rates, and maturities, cost of debt is meant to represent a blended, current estimate rather than the rate on any single loan. It is commonly estimated from the current yield to maturity on the company's outstanding debt - a market-based read on what the company would pay to borrow today - or approximated more simply as interest expense divided by average total debt, a figure that can be pulled directly from the income statement and balance sheet.
The Formula
There are two commonly cited approaches to estimating cost of debt, and analysts choose between them based on what data is available:
| Method | Formula | What it reflects |
|---|---|---|
| Yield to maturity | Current market yield on the company's outstanding, actively traded debt | A forward-looking, market-priced estimate of what the company would pay to borrow today |
| Interest expense method | Interest expense ÷ average total debt | A backward-looking, filings-based approximation of the blended rate actually paid over the period |
Once a pre-tax cost of debt is estimated by either method. It is converted to an after-tax figure for use in WACC, since interest expense is typically tax-deductible and that deductibility reduces its effective cost to the company:
After-tax cost of debt = Pre-tax cost of debt × (1 − Tax rate)
This after-tax figure, not the pre-tax rate, is the one that belongs in a WACC calculation alongside the cost of equity, each weighted by its share of the company's capital structure.
Worked Example
Hypothetical example - for education only. Suppose a company reports $40 million of interest expense for the year. Total debt was $480 million at the start of the year and $520 million at year end, for an average total debt of $500 million.
Pre-tax cost of debt (interest expense method) = $40 million ÷ $500 million = 8.0%.
If the company's effective tax rate is 21%, the after-tax cost of debt is:
8.0% × (1 − 0.21) = 8.0% × 0.79 = 6.32%
That 6.32% is the figure that would be used as the debt component of this company's WACC - lower than the 8.0% pre-tax rate because the tax deduction on interest expense reduces the effective cost of borrowing. If, instead, the company's outstanding bonds were actively traded and showed a current yield to maturity of 7.5%, an analyst might use 7.5% × (1 − 0.21) = 5.93% as an alternative, market-based estimate - the two methods can reasonably diverge, and neither is automatically the more correct one.
How Cost of Debt Is Used
Cost of debt's main use is as one of the two inputs into WACC, the discount rate commonly applied to future cash flows in a discounted cash flow valuation. Because interest is tax-deductible in most jurisdictions and dividends to equity holders are not, debt is generally the cheaper source of capital on an after-tax basis - which is part of why capital structure decisions weigh the tax benefit of debt against the added financial risk it introduces.
A company's cost of debt can also be read as a rough, market-influenced signal of perceived credit risk: all else equal, a rising yield to maturity on a company's bonds suggests the market is demanding more compensation for lending to it, though this is only one signal among many and can move for reasons unrelated to the specific company, such as broad interest-rate cycles.
Because both common estimation methods are simplifications, and because yield to maturity, tax rates, and debt levels all change over time, cost of debt is best treated as an estimate to be revisited whenever inputs move materially - not a fixed constant plugged into a model once and left alone.
Limitations and Common Mistakes
| Mistake | Why it causes problems | Better practice |
|---|---|---|
| Using pre-tax cost of debt in WACC | Overstates the effective cost of debt by ignoring the tax deductibility of interest expense. | Always convert to after-tax cost of debt before using it in a WACC calculation. |
| Using book interest expense on debt issued long ago | The interest expense method reflects historical borrowing rates, which can differ meaningfully from what the company would pay to borrow today. | Where actively traded debt exists, cross-check against current yield to maturity rather than relying on the historical rate alone. |
| Assuming yield to maturity is always available | Many companies, especially smaller ones, have no actively traded public debt to read a market yield from. | Fall back to the interest expense method, or a synthetic yield estimated from comparable credit profiles, and note the added uncertainty. |
| Ignoring changes in tax rate or debt mix | A stale tax rate or an outdated debt balance can distort the after-tax figure well after the company's actual capital structure has shifted. | Recalculate cost of debt when leverage, credit rating, or the applicable tax rate changes materially. |
Both commonly cited estimation methods are simplifications of a more complex reality - a company's true marginal cost of borrowing depends on the specific terms, collateral, and seniority of new debt it would issue today, which neither a trailing interest-expense ratio nor a single bond's yield fully captures. Treat the resulting figure as a reasonable estimate for a WACC calculation, not a precise, guaranteed number.
Frequently Asked Questions
What is the cost of debt in simple terms?
It is the effective interest rate a company pays on its borrowings - what lenders and bondholders charge it to use their money. It is commonly estimated from the current yield to maturity on the company's outstanding debt, or approximated as interest expense divided by average total debt.
Why is cost of debt used after-tax in WACC?
Interest expense is typically tax-deductible, so the government effectively subsidizes part of a company's interest bill. Using the after-tax cost of debt in a WACC calculation reflects that reduced effective cost rather than overstating what debt actually costs the company.
Is cost of debt the same as the interest rate on a company's loans?
Not exactly. A company can carry several debt instruments issued at different times and rates. Cost of debt is meant to represent a blended, current estimate of what the company would pay to borrow today, commonly proxied by yield to maturity or by dividing total interest expense by average total debt.
Which is more accurate: yield to maturity or interest expense divided by debt?
Neither is universally more accurate - each is a commonly cited approximation with tradeoffs. Yield to maturity reflects current market pricing but requires actively traded debt to observe. Interest expense divided by average total debt is easy to calculate from filings but reflects historical rates rather than what the company would pay to borrow today.
Does cost of debt apply to companies with no traded bonds?
Yes, though the estimate becomes rougher. Without a traded bond to read a yield from, analysts commonly fall back to interest expense divided by average total debt, or estimate a synthetic yield from the company's credit profile relative to similarly rated issuers - both are approximations, not precise measurements.
How does cost of debt affect a company's WACC?
WACC blends the after-tax cost of debt and the cost of equity, weighted by each source's share of the capital structure. Because debt is usually cheaper than equity and its interest is tax-deductible, a higher proportion of debt in the capital structure tends to lower WACC - up to the point where added leverage begins raising both costs due to higher perceived risk.
How should the cost of debt be estimated for a company with no traded bonds?
Common approaches use the interest rate on recent borrowings disclosed in the debt footnote, a synthetic rating derived from coverage ratios mapped to observed spreads for that rating, or the rate on a comparable company's debt. Each is an approximation. Using historical interest expense divided by average debt is a fourth approach that reflects legacy borrowing rather than current market conditions.
Why does using the coupon rate on existing debt understate the current cost?
Existing debt was issued at past rates, so its coupon reflects conditions at issuance rather than what the company would pay today. In a valuation the relevant figure is the marginal cost of new borrowing. A company with old low-coupon debt maturing into a higher-rate environment faces a rising cost that the current interest expense does not show.
How does the tax shield on interest actually work in the calculation?
Interest is generally deductible, so the after-tax cost is the pre-tax rate reduced by the tax rate, which is why the weighted calculation uses the after-tax figure. This assumes the company has sufficient taxable income to use the deduction. A loss-making company or one with limitations on interest deductibility does not receive the full shield, which the standard formula assumes it does.