Aftertax Cost of Debt Calculator
With this aftertax cost of debt calculator, you can easily calculate how much it costs a company to raise new debts to fund its assets.
After reading this article, you will understand what is the aftertax cost of debt and how to calculate the aftertax cost of debt. You will also understand how to apply the aftertax cost of debt formula to reallife situations.
What is the aftertax cost of debt?
Before we dive into the concept of the aftertax cost of debt, we must first understand what is the cost of debt and the cost of debt formula.
We define the cost of debt as the market interest rate, or yield to maturity (YTM), that the company will have to pay if it were to raise new debt from the market. Don't worry if this sounds technical, we explain in detail how you can obtain the cost of debt in the following section. Please use our bond YTM calculator and yield to maturity calculator.
However, when this concept is applied in reallife, where tax needs to be accounted for, the aftertax cost of debt is more commonly used. The main reason for this is because the interest paid on debt is often taxdeductible.
How to calculate the aftertax cost of debt using the aftertax cost of debt formula?
There is no better way to understand the concept of the aftertax cost of debt than to see it applied in real life.
To facilitate this, let's assume that we are analyzing a hypothetical company, Bill's Brilliant Barnacles, with the following information:
 The debt of Bill's Brilliant Barnacles has a credit rating of AA;
 The debt of Bill's Brilliant Barnacles has a maturity of 15 years;
 Pretax income of Bill's Brilliant Barnacles in 2020 was $1,000,000; and
 Net income of Bill's Brilliant Barnacles in 2020 was $800,000.
To calculate the aftertax cost of debt, there are 3 steps you need to follow:

Calculate the cost of debt
Determining a company's beforetax cost of debt, or the cost of debt, has always seemed difficult and complicated.
As we explained above, the cost of debt is the market interest rate, or yield to maturity (YTM), that the company will have to pay to its debtor to raise new debts from the market. However, more often than not, it is almost impossible to obtain the market interest rate of a particular company, especially when the company's debt is not publicly traded. So, how do we calculate the beforetax cost of debt of a company?
Worry not. There is still a way that we can obtain this information. And to do that, we need to know the credit rating and the maturity of the company's existing debt. In our example, the credit rating of Bill's Brilliant Barnacles' existing debt is AA and the maturity of its existing debt is 15 years.
Using these 2 pieces of information, we can estimate the company's beforetax cost of debt by comparing its debt to other publicly traded bonds with a similar credit ratings. For instance, if Charlie's Cheerful Cobblers, a company with debts of similar credit rating and maturity as Bill's Brilliant Barnacles, has an 8% yield to maturity (YTM) for its debt, we can safely assume that Bill's Brilliant Barnacles' beforetax cost of debt will be approximately 8% as well.

Calculate the marginal corporate tax rate
There are 2 inputs that you need to calculate the marginal corporate tax rate, namely the company's pretax income and the company's net income.
You can calculate the marginal corporate tax rate using the formula below:
marginal corporate tax rate = 1  (net income / pretax income)
As the corporate tax rate is applied to the pretax income, the equation above should give us the marginal corporate tax rate of Bill's Brilliant Barnacles, which is:
marginal corporate tax rate = 1  ($800,000 / $1,000,000) = 1  0.8 = 0.2 = 20%

Calculate the aftertax cost of debt
Now that we have obtained the beforetax cost of debt and the marginal corporate tax rate, it is time to calculate the aftertax cost of debt. The aftertax cost of debt can be calculated using the aftertax cost of debt formula shown below:
aftertax cost of debt = beforetax cost of debt * (1  marginal corporate tax rate)
Thus, in our example, the aftertax cost of debt of Bill's Brilliant Barnacles is:
aftertax cost of debt = 8% * (1  20%) = 6.4%
What are the benefits of calculating the aftertax cost of debt?
The benefits of finding the aftertax cost of debt (for example, with our aftertax cost of debt calculator) are:

Aftertax cost of debt assists us in making investment decisions
If you want to fund a project with debt, it is essential to make sure the aftertax cost of debt of funding the project is less than the project's rate of return. In other words, the aftertax cost of debt is the required rate of return of the project if the project is funded 100% by debt.
To earn a return from the project, the project's rate of return has to be more than the required rate of return, which is the aftertax cost of debt.

Aftertax cost of debt helps us to assess the riskiness of a company
Calculating the aftertax cost of debt is also useful in assessing the riskiness of a company. If the company's aftertax cost of debt is a lot higher than the market average, it reflects that the investors require a higher return from the company. This means that the company is riskier compared to similar companies in the market.

Aftertax cost of debt is used in calculating the weightedaverage cost of capital (WACC)
Last, but not least, the aftertax cost of debt is also an important part of calculating the WACC, which is made up of the aftertax cost of debt, cost of equity, and the company's capital structure. You can use our WACC calculator and cost of equity calculator.
In a nutshell, calculating the aftertax cost of debt is essential when assessing companies and projects. It can tell you how risky a company is and how much return you need for a project to be profitable. It is thus very crucial to understand what is the aftertax cost of debt and how to find the aftertax cost of debt.
However, the cost of debt is by no means the only metric to consider when assessing different projects and companies. So, it is recommended to form an overview of the company and ensure that every other metric is aligned before you make an investment decision.