After-tax Cost of Debt Calculator

Calculate the after-tax borrowing cost from a pre-tax debt rate and marginal tax rate, then compare the tax shield, the effective rate, and the WACC impact.

Finance planning estimate

Topic review: Michael Brennan

Small Business Finance Writer. Assigned as the finance topic reviewer for tax, debt, repayment, payroll, and business-finance calculators.

Reviewed 1 May 2026 Updated 5 May 2026 View reviewer profile Contact editorial team

Estimate the tax-shield-adjusted borrowing cost The after-tax cost of debt is the pre-tax borrowing rate reduced by the value of the interest tax shield. Finance teams use it when they compare debt financing options or build WACC assumptions.

Pre-tax cost of debt (%) Marginal tax rate (%) Debt weight in capital structure (%, optional WACC context)

Formula reference

After-tax cost of debt = pre-tax cost × (1 − tax rate).

Tax shield = pre-tax cost × tax rate.

WACC debt component = debt weight × after-tax cost of debt.

Result

4.5%

After-tax cost of debt — the effective interest expense after accounting for the tax deductibility of interest payments.

Pre-tax cost: 6%
Tax shield: 1.5%
Tax rate: 25%
WACC debt component: 1.8%

Why this number matters Interest deductibility lowers the effective cost of debt only when the business can use the deduction. That makes the after-tax figure the debt component most analysts use in WACC and capital budgeting work.

How to read this result

Interest payments are tax-deductible, reducing the effective cost of borrowing from 6% to 4.5%. The tax shield of 1.5% represents the portion of interest cost offset by the deduction.

At a debt weight of 40%, the debt side adds 1.8% to WACC before any equity-cost contribution is added.

Use the marginal corporate tax rate that actually applies to deductible interest when you want a planning estimate for valuation or WACC analysis.

After-tax interest on $1.00

$0.75

Useful when you want to explain how much of each dollar of interest still lands on the income statement after the tax shield.

Tax shield per $1.00 of interest

$0.25

Shows the theoretical share of each interest dollar offset by the marginal tax deduction when the shield is fully usable.

About this calculator

Finance planning estimate Reviewed 1 May 2026 Updated 5 May 2026

Topic review

Michael Brennan

Small Business Finance Writer. Assigned as the finance topic reviewer for tax, debt, repayment, payroll, and business-finance calculators.

Formula provenance

The methodology summary explains the formula family in use, and the source list points to the primary or authoritative references checked for this topic.

Methodology

Uses After-Tax Cost of Debt = Pre-Tax Cost × (1 − Tax Rate) to calculate the effective borrowing cost after tax deductibility, then shows the optional WACC debt contribution at the selected debt weight.

Limitations

Assumes sufficient taxable income to fully utilise the interest deduction.
Uses a single marginal tax rate and does not model blended or effective tax rates.
Does not account for tax loss carryforwards, deduction caps, Section 163(j)-style limits, or jurisdiction-specific interest-limitation rules.
Does not model changes in tax rates, fees, or refinancing costs over the debt's life.

Disclaimer

Use this result as a planning aid only. For borrowing, valuation, or tax decisions, verify the position against current official guidance or a qualified financial or tax adviser.

Primary finance sources

IRS — business interest expense deduction limitation — Official IRS overview of business interest expense deduction limits, including when the practical tax shield can be smaller than the simple formula.
NYU Stern — DCF Inputs and Cost of Capital — Damodaran lecture notes covering cost of capital inputs and the after-tax debt component used in valuation.
Wall Street Prep — Cost of Debt — Finance reference explaining pre-tax debt cost, the tax shield, and how the after-tax formula is used.
Corporate Finance Institute — Weighted Average Cost of Capital (WACC) — CFI guide showing the WACC formula and the after-tax debt term used in the blended capital-cost calculation.

Change notes

Finance change note: the formula, tax or rate assumptions, source set, disclaimer, and planning guidance for this calculator were last reviewed on 2026-05-01. Report a correction at contact@calcipedia.org; substantive rule or assumption changes refresh both the reviewed date and the updated date shown on the page.

See our calculation methodology and editorial standards, plus the editorial contact route, for how this estimate is produced and corrected.

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Corporate Finance

After-tax cost of debt explained: formula, tax shield, and WACC impact

The after-tax cost of debt is the effective interest rate a company pays on its borrowings after accounting for the tax deductibility of interest expenses.

What the after-tax cost of debt measures

Interest payments on corporate debt are tax-deductible in most jurisdictions, reducing the effective borrowing cost below the stated interest rate. The after-tax cost captures this benefit by translating the tax shield into a lower effective financing rate.

Companies and analysts use the after-tax cost of debt as the debt component in WACC calculations, which in turn drives capital budgeting and valuation decisions. It is therefore more than a bookkeeping adjustment: it changes the hurdle rate used to judge projects, acquisitions, and long-term financing choices.

Core formula

The calculation is straightforward: multiply the pre-tax borrowing rate by one minus the marginal corporate tax rate. The marginal rate matters because the benefit comes from the next deductible dollar of interest, not from a blended historical tax average.

If a company cannot use the deduction fully, the formula still shows the theoretical after-tax cost, but the real-world benefit may be smaller than the simple algebra suggests.

After-Tax Cost of Debt = Pre-Tax Cost × (1 − Tax Rate)

Where pre-tax cost is the yield to maturity or effective interest rate on the company's debt, and the tax rate is the marginal corporate rate.

WACC debt component = Debt weight × After-Tax Cost of Debt

Shows how the tax-adjusted debt cost flows into the blended capital-cost calculation.

Worked example

A company borrows at 6% and faces a 25% marginal tax rate. After-tax cost = 6% × (1 − 0.25) = 4.5%. The 1.5% tax shield reduces the effective cost from 6% to 4.5%, which is why the debt component in valuation models is usually entered after tax.

If the same borrower had a 30% marginal rate, the after-tax cost would fall to 4.2%. If the tax rate were zero, the after-tax cost would stay at 6% because there would be no deductibility benefit at all.

How to use debt weight and the WACC debt component

The after-tax cost of debt on its own is useful, but many users immediately need one more step: how much of the total hurdle rate comes from debt financing at the current capital structure. If debt is 40% of total capital and the after-tax cost is 4.5%, the debt side contributes 1.8 percentage points to WACC before the equity component is added.

That is why this page shows the WACC debt component as a separate output. It helps turn a formula answer into a planning answer by linking the tax-adjusted debt rate to the debt share actually used in valuation, capital budgeting, or financing comparisons.

How to choose the pre-tax borrowing rate

For bonds, the cleanest starting point is often the yield to maturity because it reflects the market's required return rather than just the coupon label. For loans or other debt instruments, use the effective borrowing rate the company actually pays if that is available.

When debt has been issued at a discount or premium, or when multiple borrowings carry different rates, analysts often estimate a blended pre-tax cost from the outstanding debt stack. That keeps the after-tax result closer to the funding mix used in the real capital structure.

How the tax shield affects WACC and valuation

The reason finance teams care about the after-tax cost of debt is that it feeds directly into WACC, the discount rate often used for project screening and valuation. A lower after-tax debt cost lowers the blended hurdle rate, all else equal, which can raise estimated project value in discounted cash flow work.

That does not mean debt is free capital. A lower after-tax debt cost only reflects the tax benefit on interest, not refinancing risk, covenant risk, maturity walls, or the operational risk that still comes with leverage.

The per-dollar tax-shield view can also be useful when communicating the result. If the tax shield is 25 cents per $1.00 of interest, management can quickly see why the economic borrowing cost is lower than the stated coupon or loan rate, while still remembering that the cash interest must be paid first.

Limitations

Assumes the company has sufficient taxable income to fully utilise the interest deduction. Loss-making companies, firms with carryforward losses, or entities facing deduction limits may not benefit from the full tax shield.

Does not model changes in tax rates over time, different tax rates across jurisdictions, or specific thin-capitalisation and interest-limitation rules.

Does not adjust for fees, refinancing costs, covenant pricing steps, or rate changes on floating-rate debt.

Educational estimation tool only: use it as a planning aid, not as a substitute for a valuation memo, lender review, or tax analysis.

Frequently asked questions

Why is the after-tax cost of debt lower than the pre-tax cost?

Because interest payments are tax-deductible, the government effectively subsidises part of the borrowing cost. The tax shield equals the pre-tax rate multiplied by the tax rate.

What pre-tax rate should I use?

Use the yield to maturity on the company's outstanding bonds, or the effective interest rate on its loans. Do not use the coupon rate unless the bonds trade at par.

What tax rate should I use?

Use the marginal corporate tax rate that applies to deductible interest, not a blended historical average. The marginal rate is the rate that changes the next dollar of taxable income, which is the rate that matters for the interest tax shield.

What if the company has no taxable income?

If the company cannot use the interest deduction (because it is loss-making or has no taxable income), the after-tax cost equals the pre-tax cost — there is no tax shield benefit.

How does after-tax cost of debt feed into WACC?

WACC = (E/V) × Cost of Equity + (D/V) × After-Tax Cost of Debt. The after-tax figure is used because it reflects the true economic cost of the debt component.

Why does the calculator ask for debt weight as well?

Debt weight is optional WACC context. It shows how much the after-tax debt rate contributes to the total hurdle rate at the current capital structure, which is more useful for planning than a standalone debt percentage in isolation.

Is the after-tax cost of debt the same as the effective interest rate?

Not exactly. The effective interest rate is the borrowing cost before tax, while the after-tax cost of debt is that same cost adjusted for the interest tax shield. In other words, the effective rate is the starting point and the after-tax cost is the valuation input.

Can the tax shield be smaller than the formula suggests?

Yes. The simple formula assumes the interest deduction is fully usable. If the company has losses, caps, restricted deductions, or tax attributes that block the full benefit, the practical tax shield can be smaller than the theoretical one shown by the calculator.

Should I use the same rate for floating-rate debt?

Use the current effective borrowing rate or a reasonable forward estimate if the rate changes over time. For floating-rate debt, the after-tax cost will move with the underlying reference rate, spread, and tax rate assumptions.

Is the formula the same in every country?

The algebra is the same, but the applicable tax rules are not. Different jurisdictions can limit interest deductibility, define taxable income differently, or apply specific anti-avoidance rules, so the marginal tax rate and deduction assumptions should match the country you are analysing.

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