Cost of debt is the interest a company pays on its borrowings. It is expressed as a percentage rate. In addition, cost of debt can be calculated as a before-tax rate or an after-tax rate. Because interest is deductible for income taxes, the cost of debt is usually expressed as an after-tax rate.
The formula for the cost of debt is the sum of the risk-free rate plus the credit spread times one minus the tax rate (Rf + Credit Spread)*(1 - Tax Rate).