To calculate the before-tax cost of debt, divide a company’s total annual interest expense by its total interest-bearing debt and multiply by 100. The result is the average interest rate the company is paying on its borrowings before any tax benefit from deducting that interest. A company paying $75,000 in annual interest on $1,500,000 of loans has a before-tax cost of debt of 5%.
The arithmetic is easy. The judgment is in the inputs.
The Formula
Before-Tax Cost of Debt (%) = (Total Annual Interest Expense ÷ Total Interest-Bearing Debt) × 100
Two definitions do most of the work here.
Interest expense means only the cost of borrowing. Late fees, loan origination costs, and debt issuance costs do not belong in the numerator.
Interest-bearing debt means bank loans, bonds, lines of credit, and finance lease liabilities. It does not include accounts payable, accrued wages, deferred revenue, or other operating liabilities that sit in the liabilities section of the balance sheet but carry no stated interest rate. Sweeping those in inflates the denominator and pushes the calculated cost of debt artificially low.
A Single-Loan Example
A company has one bank loan with a principal balance of $2,000,000 and pays $110,000 of interest over the year.
$110,000 ÷ $2,000,000 = 0.055 × 100 = 5.5%
That 5.5% is the effective rate. If the stated rate on the note was also 5.5%, the numbers match. If the stated rate was 6% but principal was paid down mid-year, the calculated figure will differ, and the calculated figure is the one that reflects what the debt actually cost.
When the Debt Includes Bonds, Use Yield to Maturity
The coupon rate printed on a bond is not the right number if the bond trades in the market. Prices move with interest rates and creditworthiness, so the company’s real cost is the yield to maturity, which reflects the total return an investor earns holding the bond to maturity based on today’s price.
An exact YTM has to be solved iteratively, but the standard approximation is close enough for most work:
YTM ≈ (Annual Coupon + (Face Value − Current Price) ÷ Years to Maturity) ÷ ((Face Value + Current Price) ÷ 2)
Take a bond with a $1,000 face value, a $60 annual coupon, a market price of $920, and 10 years remaining:
- Numerator: $60 + ($1,000 − $920) ÷ 10 = $60 + $8 = $68
- Denominator: ($1,000 + $920) ÷ 2 = $960
- YTM: $68 ÷ $960 ≈ 7.1%
The coupon rate is 6%, but the before-tax cost of that debt at current market prices is roughly 7.1%. The bond trades at a discount because investors are demanding a higher yield than the coupon provides, often reflecting credit risk. Research from the Federal Reserve Bank of New York decomposes corporate bond yields into a risk-free rate, expected default losses, and a risk premium, with the risk premium reaching up to 1.8% at intermediate maturities.1Liberty Street Economics (Federal Reserve Bank of New York). Estimating the Term Structure of Corporate Bond Risk Premia Use the market yield, not the original coupon.
Floating-Rate Debt
Many commercial loans and lines of credit are priced as a benchmark rate plus a fixed margin. The dominant U.S. dollar benchmark is now the Secured Overnight Financing Rate (SOFR), which stood at 3.66% in early March 2026.2FRED (Federal Reserve Bank of St. Louis). Secured Overnight Financing Rate (SOFR) A loan priced at “SOFR plus 200 basis points” currently carries a rate of about 5.66%.
For a floating-rate obligation, use the rate currently in effect, not the rate at origination. Most commercial loans reference a 30- or 90-day average of SOFR rather than a single daily reading, which smooths out day-to-day noise.3Federal Reserve Bank of New York. Options for Using SOFR in Adjustable Rate Mortgages Pull the current rate from the most recent interest statement.
Weighted Average Across Multiple Debts
Most companies borrow in more than one form. A simple average of the rates on those obligations is misleading because it treats a $200,000 equipment loan the same as a $10 million bond. Weight each rate by its share of total debt.
Suppose a company has three obligations:
- Term loan: $3,000,000 at 5.0%
- Corporate bond: $6,000,000 at 6.5% (YTM)
- Revolving credit line: $1,000,000 at 7.2%
Total debt is $10,000,000. Multiply each rate by that debt’s share of the total:
- Term loan: 0.30 × 5.0% = 1.50%
- Bond: 0.60 × 6.5% = 3.90%
- Credit line: 0.10 × 7.2% = 0.72%
Add them: 1.50% + 3.90% + 0.72% = 6.12%. That is the company’s before-tax cost of debt. A simple average of the three rates would have produced 6.23%, overstating the cost by giving the small, expensive credit line the same weight as the large bond.
Where to Find the Inputs
For a public company, both figures live in the annual financial statements. Interest expense appears as its own line on the income statement. Interest-bearing debt appears on the balance sheet, split between the current portion and long-term liabilities. Both are reported in the 10-K filed annually with the Securities and Exchange Commission.4Securities and Exchange Commission. Investor Bulletin: How to Read a 10-K
For a private company, the same information comes from internal financial statements or the corporate tax return. On Form 1120, interest expense is reported as a deduction on Line 18.5IRS. 2025 Instructions for Form 1120 – U.S. Corporation Income Tax Return Outstanding balances come from loan statements or the notes to the financials. If audited financials aren’t available, sum interest paid from individual loan statements directly.
Mistakes That Distort the Result
- Using the stated rate instead of the effective rate. If debt was issued at a discount or premium, or fees were rolled in, actual interest paid divided by actual balance outstanding is the honest figure.
- Including non-interest-bearing liabilities in the denominator. Accounts payable and accrued expenses are obligations, but they carry no interest rate, so adding them dilutes the calculation.
- Using book value for traded bonds. A bond issued at par years ago may trade at a discount or premium now; the market yield reflects current cost.
- Using an origination rate on floating-rate debt. That number goes stale the first time the benchmark resets.
- Mixing up before-tax and after-tax figures when handing the result to someone else.
Before-Tax vs. After-Tax: Which One You Need
Before-tax cost of debt is the contractual burden of the borrowing. It’s the right figure for comparing companies across different tax situations, and it’s the starting point every other debt-cost analysis builds on. It is not the number that goes into a weighted average cost of capital (WACC) model. For WACC, convert to the after-tax cost, because interest is generally deductible under federal law.6Office of the Law Revision Counsel. 26 U.S. Code 163 – Interest
After-Tax Cost of Debt = Before-Tax Cost of Debt × (1 − Tax Rate)
At a 21% federal corporate rate, a 6.12% before-tax cost becomes 6.12% × (1 − 0.21) = 4.83% after tax.
One caveat that matters for heavily leveraged companies: Section 163(j) caps the deductible business interest at 30% of adjusted taxable income plus any business interest income received. For tax years beginning after December 31, 2024, adjusted taxable income is calculated with depreciation, amortization, and depletion added back, which raises the cap for capital-intensive businesses. Small businesses with average annual gross receipts of $31 million or less (the inflation-adjusted 2025 threshold) are generally exempt.7IRS. Questions and Answers About the Limitation on the Deduction for Business Interest Expense When the limit bites, disallowed interest carries forward but doesn’t reduce the current year’s tax shield, so the effective after-tax cost of debt sits above what the standard formula suggests. The before-tax figure is unaffected either way.