π³ After-Tax Cost of Debt Calculator
What borrowing really costs after the tax deduction.
True annual cost of the debt
Estimates for planning only, not financial advice.
The after-tax cost of debt is what borrowing truly costs once the tax deduction on interest is counted: pre-tax rate times one minus the tax rate. At a 6.5 percent interest rate and a 21 percent tax rate, the after-tax cost is 5.135 percent, because the deduction saves 1.365 points. This is the number that belongs in WACC and in every honest comparison of financing options, since the pre-tax rate overstates what debt really costs a taxpaying borrower.
How to Use This Calculator
Enter the pre-tax interest rate on the loan or bond and your marginal tax rate (federal plus state combined). Optionally enter the debt amount to see the annual interest, the tax saving, and the net cost in dollars. The calculator also shows the muni-equivalent: the taxable yield you would need to match a tax-exempt return.
Enter the pre-tax rate: the loan's interest rate or the bond's yield to maturity.
Enter your marginal tax rate, not the effective rate. The deduction shelters the next dollar of interest, so the marginal rate is the right one.
Read the after-tax rate and the dollar tax shield. Use the after-tax rate in WACC and project hurdle rates, never the pre-tax rate. For loan shopping on fees as well as rate, see our APR calculator.
The Formula
The formula is one line because the economics are one line: interest reduces taxable income dollar for dollar, so the after-tax cost is the rate minus the tax saving. On $1 million of debt at 6.5 percent, annual interest is $65,000, the tax shield at 21 percent is $13,650, and the net cost is $51,350, which is 5.135 percent of the debt. The tax-equivalent line runs the same logic backward for muni investors.
Worked Examples
Corporate borrower: $1,000,000 term loan at 6.5 percent, 21 percent federal rate. After-tax cost: 6.5% x (1 - 0.21) = 5.135%. Annual interest $65,000, tax saving $13,650, net cost $51,350. In a 6 percent state (roughly 25.7 percent combined), the cost falls to 4.83 percent and the shield rises to $16,731. State taxes matter to the real cost.
Muni investor: a 3.8 percent tax-exempt muni yield at a 24 percent marginal bracket equals a taxable-equivalent yield of 3.8 / (1 - 0.24) = 5.00 percent. A taxable corporate bond yielding 4.8 percent loses to the muni on an after-tax basis despite the higher headline number. This is the entire muni market in one calculation: lower yields winning after tax in high brackets.
When the Shield Shrinks or Vanishes
The formula assumes every interest dollar is deductible, and three situations break that. Companies with net operating losses pay no tax, so T is effectively zero until profits return; their cost of debt is the full pre-tax rate. The 30-percent-of-adjusted-taxable-income cap on business interest deductions bites highly leveraged firms, making marginal debt more expensive than the formula says. And individuals mostly lost the deduction: with today's large standard deduction, most homeowners do not itemize, so their mortgage costs the full rate.
| Borrower | Typical T | 6.5% debt really costs |
|---|---|---|
| C-corp, federal only | 21% | 5.14% |
| C-corp, federal + 6% state | ~25.7% | 4.83% |
| Itemizing homeowner | 24% | 4.94% |
| Non-itemizer / loss company | 0% | 6.50% |
Using It in Decisions
The after-tax cost of debt is the debt leg of WACC, the hurdle rate for projects, and the right comparator when choosing between debt and equity financing. A project earning 7 percent clears a 5.14 percent after-tax debt cost comfortably but might not clear an 8 percent cost of equity; that spread is the whole capital-structure game. For refinancing decisions, combine this with fee analysis: our refinance break-even calculator prices the fee-versus-rate trade, and the loan amortization calculator shows the payment schedule behind any rate you enter here.
One caution: the tax shield is only as reliable as the profits sheltering it. In a downturn, the company paying 6.5 percent pre-tax with no taxable income is paying 6.5 percent, full stop, at exactly the moment cash is scarcest. Stress-test financing decisions at T = 0 as well as at the statutory rate; the pessimistic case is the one that shows up in recessions.
When the Tax Shield Disappears
The after-tax cost formula assumes the company actually pays taxes. Firms with net operating losses, heavy tax credits, or operations in low-tax jurisdictions get little or no shield, and their true cost of debt sits close to the pre-tax rate. Startups burning cash should never use the after-tax number in their hurdle rates; the deduction is a future maybe, not a current benefit.
For individuals, the shield is narrower than it looks. Mortgage interest only helps if you itemize, which fewer households do since the standard deduction roughly doubled; if you take the standard deduction, your mortgage costs the full note rate. Student loan interest has income caps and a $2,500 annual limit. The honest approach is marginal: compute the deduction you actually get on the next dollar of interest, not the statutory rate. A 22 percent bracket means nothing if the standard deduction already covers you.
Estimates for planning only, not financial advice.
Frequently Asked Questions
What is the after-tax cost of debt?
The true cost of borrowing after the tax deduction on interest: pre-tax rate x (1 - tax rate). A company borrowing at 6.5 percent with a 21 percent tax rate has an after-tax cost of 5.135 percent, because the interest deduction saves 1.365 points. It is the debt input to WACC, the weighted average cost of capital.
What is the after-tax cost of debt formula?
After-tax cost of debt = rd x (1 - T), where rd is the pre-tax interest rate and T is the marginal tax rate. With rd = 6.5% and T = 21%, the math is 6.5% x 0.79 = 5.135%. Use the marginal rate, not the effective rate, because the deduction applies to the next dollar of interest.
Why is debt cheaper after tax?
Because interest is tax-deductible for most businesses. Every dollar of interest reduces taxable income by a dollar, saving T cents of tax. The government effectively subsidizes 21 cents of every interest dollar at the federal corporate rate, which is why debt is structurally cheaper than equity and why companies borrow.
Do individuals get an after-tax cost of debt?
Sometimes. Mortgage interest is deductible for itemizers, so a 7 percent mortgage at a 24 percent marginal rate costs 5.32 percent after tax. But since the 2017 tax law raised the standard deduction, most homeowners no longer itemize and get no deduction, making their cost of debt the full pre-tax rate. Business interest is broadly deductible; personal interest mostly is not.
What tax rate should I use?
The marginal combined rate: federal plus state, adjusted for deductibility interactions. A company in the 21 percent federal bracket in a 6 percent state faces roughly 25.7 percent combined (21% + 6% x 79%). If the company has losses and pays no tax, the effective T is zero and there is no tax shield until profits return.
How does this relate to WACC?
WACC blends the after-tax cost of debt with the cost of equity by their market-value weights: WACC = (E/V) x Re + (D/V) x Rd x (1 - T). Debt gets the tax adjustment; equity does not, because dividends are not deductible. Using the pre-tax rate in WACC overstates the cost of capital and can kill good projects.
What is the tax-equivalent yield on munis?
The mirror image: a tax-exempt muni yield divided by (1 - T) gives the taxable yield you would need to match it. A 3.8 percent muni at a 24 percent bracket equals a 5.00 percent taxable yield (3.8 / 0.76). In high brackets, munis often beat taxable bonds on an after-tax basis even with lower headline yields.
Does the tax shield have a limit?
Yes. US tax law caps business interest deductions at 30 percent of adjusted taxable income for many companies, so highly leveraged firms cannot deduct all their interest. Unused deductions carry forward. If the cap binds, the marginal after-tax cost of debt rises toward the pre-tax rate on the excess interest.
After-tax cost of debt vs APR: which for decisions?
Different tools. APR measures what a loan costs you all-in including fees; after-tax cost of debt measures what it costs after the tax benefit. For a business choosing between financing options, use after-tax cost of debt. For a consumer comparing loan offers, use APR from our <a href="/finance/apr-calculator">APR calculator</a>, since personal interest usually is not deductible.
Can the after-tax cost of debt be higher than the pre-tax rate?
No, as long as the tax rate is positive. Multiplying by (1 - T) always reduces the rate when T is between 0 and 100 percent. The only way debt costs the full pre-tax rate is T = 0: no taxable income, no deduction, no shield.