Cost of Debt: How to Calculate It and Why It Matters

The cost of debt is the effective rate a company pays to borrow — and it's a key building block of the cost of capital. Alongside the cost of equity, it feeds into the weighted average cost of capital (WACC) used to value businesses and appraise investments. Crucially, because interest is usually tax-deductible, the after-tax cost of debt is what really matters. This guide explains what the cost of debt is, how to calculate it, the tax effect, and why it sits at the heart of corporate finance.

What is the cost of debt?

The cost of debt is the return a company's lenders require — effectively the interest rate it pays on its borrowings, whether bonds, loans or other debt. Unlike the cost of equity, which has to be estimated, the cost of debt is more observable because debt carries contractual interest. It reflects the lenders' assessment of the company's credit risk: a riskier borrower pays a higher rate. Understanding it is essential because debt is usually a cheaper source of finance than equity, partly because lenders rank ahead of shareholders and partly because of the tax treatment.

How to calculate the cost of debt

There are two common ways to estimate the cost of debt, depending on the type of debt:

• Yield to maturity (YTM) — for traded debt such as bonds, the cost of debt is best represented by the yield to maturity: the total return an investor earns if they hold the bond to maturity, reflecting both the interest payments and any difference between the current price and the redemption value. The YTM, not just the coupon rate, is the true cost.
• Interest rate on borrowings — for bank loans and similar debt, the cost is the effective interest rate the company pays.

In both cases, you're trying to capture the real, current cost of the company's debt — not necessarily the historical rate on the books.

The tax shield: pre-tax vs after-tax cost

This is the part that catches people out. Because interest is generally a tax-deductible expense, borrowing reduces a company's tax bill — an effect known as the tax shield. So the cost of debt that matters for WACC is the after-tax cost of debt:

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

For example, if a company's pre-tax cost of debt is 6% and the tax rate is 25%, the after-tax cost is 6% × (1 − 0.25) = 6% × 0.75 = 4.5%. The tax deductibility of interest is a big reason debt is often a cheaper source of finance than equity.

Why the cost of debt matters

The cost of debt is important for several reasons. It's a component of WACC, the blended cost of capital used as the discount rate in valuation and investment appraisal — get it wrong and your valuations are wrong. It informs financing decisions, helping a company judge the cost of taking on more debt. And it's central to thinking about capital structure — the mix of debt and equity — because debt's lower cost is balanced against the financial risk that rising debt brings.

A note on risk

While debt is cheaper than equity, that doesn't mean more debt is always better. As a company borrows more, the risk of financial distress rises, which can push up the cost of both debt and equity and eventually outweigh the tax-shield benefit. So the cost of debt has to be considered alongside the risks of gearing, not in isolation — a theme at the heart of capital structure theory.

Frequently asked questions

What is the cost of debt?

The effective rate a company pays to borrow — the return its lenders require, reflecting its credit risk. It's more observable than the cost of equity because debt carries contractual interest.

How do you calculate the cost of debt?

Using the yield to maturity for traded debt like bonds, or the effective interest rate for loans — then adjusting for tax to get the after-tax cost.

What is the after-tax cost of debt?

Pre-tax cost of debt × (1 − tax rate). Because interest is tax-deductible, the after-tax cost — which is what's used in WACC — is lower than the pre-tax cost.

Is the coupon rate the same as the cost of debt?

Not necessarily. For a bond, the true cost of debt is the yield to maturity, which reflects the bond's current price as well as its coupon — so it can differ from the coupon rate.

Why is debt cheaper than equity?

Because lenders rank ahead of shareholders (lower risk, lower required return) and because interest is tax-deductible, creating a tax shield that reduces the effective cost.

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