After-Tax Cost of Debt

Understanding the After-Tax Cost of Debt

The after-tax cost of debt is the actual net cost a company pays to borrow funds. Because interest payments on corporate debt are generally tax-deductible in many jurisdictions, the corporate tax rate acts as a tax shield, reducing the effective interest expense.

For businesses and investors, calculating the after-tax cost of debt is critical. It is a key input in determining the Weighted Average Cost of Capital (WACC), which represents a firm's average cost to finance its assets. It is often analyzed alongside the Cost of Equity Calculator to compare debt vs equity financing.

The After-Tax Cost of Debt Formula

The standard formula to calculate the after-tax cost of debt is:

\[\text{After-Tax Cost of Debt} = \text{Pre-Tax Cost of Debt} \times (1 - T)\]

Where:

• Pre-Tax Cost of Debt: The interest rate a company pays on its debt before tax deductions.
• T: The marginal corporate tax rate.

Example Calculation

Suppose a business borrows $100,000 at a 10% interest rate (Pre-Tax Cost of Debt). The corporate tax rate is 21%.

The interest expense is $10,000 per year. Since interest is tax-deductible, it reduces taxable income by $10,000, saving the company $2,100 in taxes ($10,000 * 21%).

The net interest paid is:

\[\$10,000 - \$2,100 = \$7,900\]

The effective interest rate (After-Tax Cost of Debt) is:

\[10\% \times (1 - 0.21) = 7.9\%\]