Debt constant, also called the mortgage constant or loan constant, is annual debt service divided by the total loan amount, expressed as a percentage. Per Wall Street Prep and PropertyMetrics, it captures both the interest rate and the amortization schedule in one number, giving the yearly cost of a loan as a percent of its original balance.
How Is the Debt Constant Calculated?
The debt constant is calculated by dividing annual debt service by the original loan amount, then multiplying by 100. The formula is Debt Constant = (Annual Debt Service / Loan Amount) x 100. Per PropertyMetrics and Wall Street Prep, it folds the interest rate and the amortization period into a single percentage, so loans with different terms can be compared directly.
Its behavior follows the loan structure. On an interest-only loan the debt constant equals the interest rate, because no principal is repaid. On an amortizing loan it is higher than the rate, since each payment retires some principal on top of interest. The debt constant applies to fixed-rate loans, where the payment does not change over time.
Input | Definition |
Annual debt service | The sum of twelve monthly principal and interest payments |
Loan amount | The original principal balance of the loan |
Debt constant | Annual debt service divided by loan amount, as a percentage |
A defining property: the debt constant is always at least as large as the interest rate. The gap between them is the share of the original balance repaid as principal in the first year. A shorter amortization widens that gap and raises the constant, even when the interest rate is unchanged.
Why the Debt Constant Matters
The debt constant matters because it links financing cost to property yield in one comparison. Per the band of investment method used in appraisal, the mortgage constant (denoted Rm) is weighted with the equity return to derive a capitalization rate. When a property's cap rate exceeds its debt constant, leverage is positive and each borrowed dollar adds to equity return.
For an operator, the constant is a fast screen. If a deal caps at 7% and the debt constant is 7.19%, leverage is slightly negative and debt is dragging on returns before any other factor is considered. Comparing cap rate to debt constant, rather than cap rate to interest rate, is the correct test because it accounts for the principal an amortizing loan forces the borrower to repay.
Example
A $10,000,000 loan carries a 6% fixed rate and a 30-year amortization. The level monthly payment is $59,955, so annual debt service is $59,955 times twelve, or $719,461. Divide annual debt service by the loan amount: $719,461 divided by $10,000,000 equals 0.0719, or a 7.19% debt constant.
Step | Calculation | Result |
Loan amount | Given | $10,000,000 |
Monthly payment | 6%, 30-year amortization | $59,955 |
Annual debt service | $59,955 x 12 | $719,461 |
Debt constant | $719,461 / $10,000,000 | 7.19% |
Note the spread. The interest rate is 6%, but the debt constant is 7.19%, and the 1.19-point difference is the principal repaid in year one as a share of the original balance. If this property caps at 7%, leverage is negative, because 7% is below the 7.19% constant. If it caps at 8%, leverage is positive.
Variations and Edge Cases
The debt constant moves with the interest rate and the amortization term, and a few structures make it behave differently. The table below shows how.
Variant | Effect on debt constant |
Interest-only loan | Debt constant equals the interest rate exactly |
Shorter amortization | Higher constant; more principal repaid each year |
Longer amortization | Lower constant, closer to the interest rate |
Floating rate | Constant is not fixed; recompute as the rate resets |
Higher interest rate | Higher constant, all else equal |
The common error is comparing a property's cap rate to the loan's interest rate to judge leverage. On an amortizing loan that understates the true annual cost, because principal repayment is a real cash outflow. The debt constant is the correct benchmark, and it is always higher than the rate on any amortizing loan.
Debt Constant vs Cap Rate
Debt constant is often confused with capitalization rate, but they describe different sides of the deal. Debt constant is annual debt service divided by the loan amount, a cost of financing. Cap rate is net operating income divided by property value, a measure of unlevered yield. One prices the debt; the other prices the asset.
The comparison between them is the leverage test. When cap rate is above the debt constant, borrowing adds to equity return, a condition called positive leverage. When cap rate is below the debt constant, leverage is negative and debt reduces return. This single comparison is why both figures belong side by side in underwriting.
Frequently Asked Questions
What is the debt constant formula?The debt constant formula is annual debt service divided by the loan amount, multiplied by 100 to express it as a percentage. A $10,000,000 loan with $719,461 of annual debt service has a debt constant of 7.19%. It captures interest and amortization in one number.
Is the debt constant the same as the interest rate?No. The debt constant equals the interest rate only on an interest-only loan. On an amortizing loan the debt constant is higher than the interest rate, because each payment repays principal in addition to interest. A 6% loan amortizing over 30 years has a debt constant of about 7.19%.
How does the debt constant relate to positive leverage?Leverage is positive when a property's cap rate is higher than the loan's debt constant, meaning each borrowed dollar earns more than it costs. When the cap rate falls below the debt constant, leverage is negative and debt drags on equity return.
Related Terms
Debt Service
Amortization
Net Operating Income
Debt Service Coverage Ratio
Debt Yield