Calculate the debt constant from annual interest rate and number of annual payments, using the standard loan amortization formula.
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Debt Constant Formula
The debt constant is the annual loan payment as a fraction of the original loan amount. It is commonly used for amortizing debt, where each annual payment includes both interest and principal.
- DC = debt constant, expressed as a decimal
- r = annual interest rate as a decimal
- n = total number of annual payments
To enter the interest rate, use the annual percentage rate. For example, enter 6 for 6%. The calculator converts that to 0.06 before applying the formula.
The calculator is set up to solve for the debt constant when you enter the annual interest rate and the number of annual payments. Solving directly for the interest rate requires an iterative method, and solving directly for the number of payments requires logarithmic rearrangement, so those fields are not calculated by this version.
Typical Debt Constants by Interest Rate and Term
The table below shows approximate annual debt constants for common interest rates and amortization periods. Values are decimals, so 0.0872 means the annual debt service is about 8.72% of the original loan amount.
| Annual Interest Rate | 10 Payments | 15 Payments | 20 Payments | 30 Payments |
|---|---|---|---|---|
| 5% | 0.1295 | 0.0963 | 0.0802 | 0.0651 |
| 6% | 0.1359 | 0.1030 | 0.0872 | 0.0726 |
| 7% | 0.1424 | 0.1098 | 0.0944 | 0.0806 |
| 8% | 0.1490 | 0.1168 | 0.1019 | 0.0888 |
Debt Constant Interpretation
| Debt Constant | Annual Debt Service per $100,000 Borrowed |
|---|---|
| 0.0600 | $6,000 per year |
| 0.0750 | $7,500 per year |
| 0.0900 | $9,000 per year |
| 0.1200 | $12,000 per year |
Example Debt Constant Calculations
Example 1: 6% interest with 20 annual payments
Suppose the annual interest rate is 6% and the loan is repaid over 20 annual payments.
The debt constant is approximately 0.087185. That means annual debt service is about 8.7185% of the original loan amount.
Example 2: 7% interest with 15 annual payments
Suppose the annual interest rate is 7% and the loan is repaid over 15 annual payments.
The debt constant is approximately 0.109795. On a $100,000 loan, that equals about $10,979.50 in annual debt service.
FAQ
What does a debt constant tell you?
A debt constant tells you the annual payment required to amortize a loan, stated as a percentage of the original loan balance. For example, a debt constant of 0.085 means the annual debt service is 8.5% of the loan amount.
Is the debt constant the same as the interest rate?
No. The interest rate only measures the cost of borrowing. The debt constant includes both interest and principal repayment. Because of that, the debt constant is usually higher than the interest rate for an amortizing loan.
How do you use the debt constant to find annual debt service?
Multiply the original loan amount by the debt constant.
For example, if the loan amount is $500,000 and the debt constant is 0.087185, the annual debt service is $43,592.50.
