After-Tax Cost of Debt Calculator

Enter your pre-tax interest rate, tax rate, debt amount, and term to calculate your effective after-tax borrowing cost, tax shield benefit, and total net interest expense.

Created by Luis GonzalezLast updated: 2026-06-08

How to Use This Calculator

1
Enter Your Debt & Tax Details
Input the pre-tax interest rate (nominal rate), your marginal tax rate, total debt amount (principal balance), and remaining debt term in years.
2
Review Your Results
The calculator displays After-Tax Cost of Debt, Tax Shield Rate, Annual Tax Savings, Total Tax Savings Over Term, Net Interest Cost, and Effective Rate Reduction. The Insights card shows tax shield value, WACC impact, and refinancing analysis.

Example Calculation

A business evaluates a $500,000 loan at 6% interest over 10 years with a 30% corporate tax rate.

Pre-Tax Cost of Debt

Tax Rate

30%

Total Debt Amount

$500,000

Debt Term

10 yrs

Results

After-Tax Cost of Debt

4.20%

Tax Shield Rate

1.80%

Annual Tax Savings

$9,000

Total Tax Savings Over Term

$90,000

Net Interest Cost Over Term

$210,000

Effective Rate Reduction

30.0%

Insights card shows $0.

Tips

Use Marginal Tax Rate, Not Effective

The tax shield applies at the marginal rate (rate on your last dollar of income). In the example, 30% marginal saves $9,000/yr. Using a 22% effective rate would understate the shield at $6,600/yr — a $24,000 difference over 10 years.

Compare After-Tax Cost to Equity Cost

In the example, 4.20% after-tax debt is much cheaper than typical equity cost (8-12%). If your cost of equity is 10%, each dollar financed with this debt at 4.20% vs. equity saves 5.80% annually — a strong argument for leveraging the tax shield.

Evaluate Refinancing at Lower Rates

A 1% reduction in the example (6% to 5%) saves $5,000/yr in gross interest and $3,500/yr after-tax. Over 10 years, that's $35,000 in after-tax savings — significant on $500,000 principal.

The After-Tax Cost of Debt Calculator reveals the true borrowing cost after the tax shield. With $500,000 at 6% pre-tax and a 30% tax rate, the after-tax cost drops to 4.20%. The 1.80% tax shield rate generates $9,000/yr in savings ($90,000 over 10 years), reducing net interest cost from $300,000 gross to $210,000 — a 30% reduction through tax deductibility.

The After-Tax Cost of Debt Formula

The calculation adjusts the nominal interest rate by the tax rate, reflecting the government effectively subsidizing a portion of your interest expense.

After-Tax Cost of Debt = Pre-Tax Rate x (1 - Tax Rate / 100)
Tax Shield Rate = Pre-Tax Rate - After-Tax Rate
Annual Interest = Debt Amount x (Pre-Tax Rate / 100)
Annual Tax Savings = Annual Interest x (Tax Rate / 100)
Total Tax Savings = Annual Tax Savings x Debt Term
Net Interest Cost = (Annual Interest x Debt Term) - Total Tax Savings

The key: at a 30% tax rate, every $1 of interest costs only $0.70 after the tax deduction. Higher tax brackets amplify the shield — at 35%, each $1 costs only $0.65.

💡 To see how mortgage interest provides a similar tax benefit, our Tax Benefits of Home Ownership Calculator illustrates deductions available for homeowners.

Worked Example: A Business's True Debt Cost

A company evaluates the true cost of its business loan.

Inputs:

Pre-Tax Cost of Debt: 6%
Tax Rate: 30%
Total Debt Amount: $500,000
Debt Term: 10 years

Step-by-step:

After-Tax Cost of Debt: 6% x (1 - 0.30) = 4.20%
Tax Shield Rate: 6% - 4.20% = 1.80%
Annual Interest Expense: $500,000 x 6% = $30,000
Annual Tax Savings: $30,000 x 30% = $9,000
Total Tax Savings (10 yr): $9,000 x 10 = $90,000
Net Interest Cost: $300,000 - $90,000 = $210,000

The tax shield saves $90,000 over the loan term — equivalent to 3 years of interest payments at the after-tax rate.

💡 For broader capital structure analysis, our WACC Calculator uses the after-tax cost of debt alongside equity costs to determine your blended cost of capital.

Tax Shield Impact by Tax Bracket

The tax shield's value scales directly with your tax rate:

Tax Rate	After-Tax Cost (6% Pre-Tax)	Annual Savings ($500K)	10-Year Savings
15%	5.10%	$4,500	$45,000
21%	4.74%	$6,300	$63,000
25%	4.50%	$7,500	$75,000
30%	4.20%	$9,000	$90,000
35%	3.90%	$10,500	$105,000

At the U.S. corporate rate of 21%, the after-tax cost is 4.74% with $63,000 in savings over 10 years. Higher marginal rates (state taxes can push effective rates to 25-30%) significantly amplify the benefit.

Frequently Asked Questions

How does the tax shield reduce the cost of debt?

Interest expense is tax-deductible, so every $1 of interest at a 30% tax rate saves $0.30 in taxes. In the example, $30,000/yr interest generates $9,000/yr in tax savings, reducing the effective rate from 6% to 4.20%. Over 10 years, the $90,000 total tax savings reduces gross interest cost of $300,000 to a net $210,000 — a 30% reduction.

How does after-tax cost of debt affect WACC?

After-tax cost of debt is a direct input to Weighted Average Cost of Capital (WACC). In the example, using 4.20% after-tax cost of debt instead of 6% pre-tax in the WACC formula reduces the weighted debt component by 30%. If debt is 40% of capital structure, this lowers overall WACC by 0.72 percentage points — which can significantly increase company valuation in DCF models.

Does Section 163(j) limit the tax shield benefit?

Yes, IRS Section 163(j) limits net business interest deductions to 30% of adjusted taxable income (EBITDA through 2025, then EBIT). In the example, the $30,000 annual interest must be less than 30% of the company's adjusted taxable income to be fully deductible. If the company earns at least $100,000 in adjusted income, the full $9,000/yr shield applies. Below that threshold, unused deductions carry forward.