Loan Constant Calculator
Calculate the loan constant (mortgage constant) and compare commercial real estate loan options at a glance.
The loan constant, also called the mortgage constant or K, is the annual debt service on a loan expressed as a percentage of the total loan amount. It rolls the interest rate and amortization schedule into a single number, which is exactly what borrowers care about: how much does this loan cost per year, per dollar borrowed. This calculator computes the loan constant, the annual debt service, and the monthly payment for any commercial real estate loan, so brokers and investors can compare lender quotes and analyze leverage in seconds.
Calculate Your Loan Constant
What Is the Loan Constant?
The loan constant is annual debt service divided by the original loan amount. It tells you, as a percentage, how much of the loan principal you pay back each year in combined principal and interest.
For an interest-only loan, the loan constant equals the interest rate, because there is no principal paydown. For an amortizing loan, the loan constant is always higher than the interest rate, because each payment includes a principal component. The longer the amortization, the closer the loan constant gets to the interest rate. The shorter the amortization, the higher the loan constant.
The loan constant is the single number that captures the full annual cost of debt as a percentage of the loan. Two loans at 6.5 percent can have very different loan constants if one amortizes over 25 years and the other over 30. The 30-year loan will have a lower loan constant and lower annual debt service, even though the rate is identical.
How to Calculate the Loan Constant
Annual Debt Service = Monthly Payment × 12
Monthly Payment = L × [r(1+r)^n] / [(1+r)^n − 1]
Where:
L = Loan amount
r = Monthly interest rate (annual rate / 12)
n = Total number of months (years × 12)
Worked Example
A $5 million loan at 6.5 percent with 25-year amortization. Monthly rate is 6.5 percent divided by 12, or 0.005417. Total months is 300. Plugging into the amortization formula gives a monthly payment of $33,760.36. Annual debt service is $405,124.32. Divided by the $5 million loan amount, the loan constant is 8.10 percent.
That 8.10 percent figure is what the borrower owes annually as a fraction of the loan, regardless of whether you call it interest or principal.
Loan Constant vs. Cap Rate: Leverage Analysis
The loan constant becomes most useful when compared to the property's cap rate. The relationship between the two determines whether the deal has positive or negative leverage.
| Scenario | Relationship | Implication |
|---|---|---|
| Positive leverage | Cap Rate > Loan Constant | Debt is accretive to cash-on-cash returns. Adding leverage increases unlevered yield. |
| Break-even leverage | Cap Rate = Loan Constant | Debt is neutral. Cash-on-cash equals the cap rate regardless of leverage. |
| Negative leverage | Cap Rate < Loan Constant | Debt drags on cash-on-cash. The deal earns less per dollar of equity than it would unlevered. |
If you buy a property at a 7.5 percent cap and finance it with a loan at an 8.1 percent loan constant, the deal has negative leverage. Year-one cash flow per dollar of equity will be lower than the cap rate, and the only path to a positive return is rent growth, expense management, or cap rate compression at exit.
If the same property had a cap rate of 9 percent against the same 8.1 percent loan constant, the deal would have positive leverage. Every dollar of debt would amplify cash-on-cash return.
Negative leverage is not always a deal-killer. Many investors accept negative leverage in year one if they can underwrite NOI growth that flips the equation by year two or three. The loan constant just tells you whether you are starting in a hole that needs to be filled by future cash flow.
How Brokers Use the Loan Constant
Loan constant is one of the fastest tools for comparing financing options. When you have term sheets from three different lenders at different rates and amortization schedules, the rate alone does not tell you which loan has the lowest annual cost. The loan constant does.
| Lender | Rate | Amortization | Loan Constant |
|---|---|---|---|
| Lender A | 6.25% | 25 years | 7.92% |
| Lender B | 6.75% | 30 years | 7.78% |
| Lender C | 6.50% | 25 years | 8.10% |
Lender B has the highest interest rate but the lowest loan constant because of the longer amortization. For a borrower focused on year-one cash flow, Lender B is the cheapest option, even though the rate looks worse on paper. For a borrower focused on long-term equity build-up, Lender A's faster amortization may be more attractive despite the slightly higher annual cost.
Loan constant also feeds directly into DSCR analysis. If you know NOI and the loan constant, you can solve for the maximum loan amount the property supports at any DSCR threshold. That makes loan constant a useful lever in deal sizing.
Loan Constant in Deal Sizing
To find the maximum loan amount a property supports at a given DSCR, the formula is:
For a property with $400,000 in NOI, a lender requirement of 1.25x DSCR, and a loan constant of 8.10 percent, the maximum supportable loan is $400,000 divided by (1.25 multiplied by 0.0810), or about $3.95 million. If the broker can find a lender with a lower loan constant (lower rate or longer amortization), the same property supports a larger loan at the same DSCR.
Loan Constant vs. Other Underwriting Metrics
| Metric | Formula | What It Measures |
|---|---|---|
| Loan Constant | Annual Debt Service / Loan Amount | Annual debt cost per dollar borrowed |
| DSCR | NOI / Annual Debt Service | Income coverage of debt payments |
| Debt Yield | NOI / Loan Amount | Return on loan amount, rate-independent |
| Cap Rate | NOI / Property Value | Unlevered return on property value |
| Mortgage Calculator | n/a | Full payment schedule and totals |
Each metric answers a different question. Loan constant compares loans. DSCR confirms the property can pay. Debt yield shows lender return independent of rate. Cap rate measures unlevered property performance. Brokers who fluently use all four can structure smarter deals and explain trade-offs clearly to clients. For a deeper dive into how these metrics fit together, see Janover Pro's guide on commercial loan products.
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This content is for informational and educational purposes only and does not constitute financial, legal, tax, or investment advice. Janover Pro is a technology platform that connects commercial mortgage brokers with lenders. Janover Pro is not a lender and does not make lending decisions. Loan terms, rates, eligibility, and availability are determined by individual lenders and are subject to change without notice. Consult qualified financial and legal professionals before making financing decisions.
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