Loan Constant Calculator
Loan Constant Calculator
Loan Breakdown
| Loan Amount | $500,000 |
| Interest Rate | 6.50% |
| Amortization Term | 30 years (360 periods) |
| Monthly Payment | $3,160.34 |
| Annual Debt Service | $37,924 |
| Loan Constant (K) | 7.585% |
| Constant per $1,000 | $75.85/yr |
| Total Payments (full term) | $1,137,722 |
| Total Interest | $637,722 |
| Origination Fee (1%) | $5,000 |
| Effective Constant (after fee) | 7.661% |
| Max Loan @ Target DSCR | $1,253,030 |
| Actual DSCR (on $120,000 NOI) | 3.16x |
Summary Statistics
What Is a Loan Constant?
The loan constant — also called the mortgage constant or debt service constant — is the annual amount of principal and interest required to repay a loan, expressed as a percentage of the original loan amount. If a $500,000 loan at 6.5% over 30 years requires $37,927 of annual payments, its loan constant is 7.59%. Because the constant bundles the interest rate together with the repayment schedule, it answers a question the rate alone cannot: how many dollars of debt service does each dollar of debt cost per year? Commercial real estate investors and lenders use it constantly, most often to turn net operating income into a defensible loan amount. The constant also lets you compare loans with different rates and terms on a single scale — a 15-year loan at 5.75% may carry a higher annual payment than a 30-year loan at 6.75%, and the constant makes that trade-off immediately visible. This calculator computes the exact constant for any loan amount, rate, and amortization schedule, then extends the math to the two questions that matter most in practice: what your effective cost is after an origination fee, and how much debt your property's income can actually support.
How the Loan Constant Is Calculated
Every fully amortizing loan payment follows the same standard formula: the periodic payment equals the loan amount times the periodic interest rate, divided by one minus the discount factor raised to the negative number of payments. In plain terms, the payment is the amount that, at the quoted rate, reduces the balance to exactly zero after the final period. The calculator converts your annual interest rate into a periodic rate by dividing by the payment frequency — 12 for monthly, 4 for quarterly, or 1 for annual schedules — then multiplies the number of years by that same frequency to find the total number of payments. The annual debt service is simply the periodic payment multiplied by the payments per year, and the loan constant divides that annual debt service by the original loan amount. Because the constant is a ratio of annual payments to balance, it behaves predictably: it falls as terms lengthen (principal is repaid more slowly), and it rises with the interest rate. Two derived figures extend the analysis. First, an origination fee reduces the proceeds you actually receive while leaving the required payments unchanged, so the effective constant divides annual debt service by the after-fee proceeds — the true annual cost per dollar of funding in hand. Second, lenders underwrite income-producing property by requiring net operating income to exceed annual debt service by a target coverage margin, typically 1.20 to 1.30 times. Since annual debt service equals loan amount times the effective constant, the maximum loan is the net operating income divided by the product of the target coverage ratio and the effective constant. The calculator reports both your actual coverage ratio on the income you entered and the maximum loan that income supports.
Understanding Your Results
The primary result is the loan constant itself, expressed as a percentage and as an annual dollar figure per $1,000 borrowed. A constant below roughly 9% reflects today's typical long-term mortgage economics; constants climb toward 11–13% on shorter terms or higher rates, and a constant above 12% signals a heavy annual debt burden relative to the amount borrowed. The breakdown table walks through every component — periodic payment, annual debt service, total payments, and total interest over the full term — so you can see exactly what drives the number. When you expand Advanced Options and add a net operating income figure, the true power of the constant appears: the calculator tells you the maximum loan a property can carry at your target coverage ratio and your actual coverage at the income level you entered. That single output is what lenders compute when they size a loan, which makes this tool useful both for borrowers checking what a lender may offer and for investors comparing how much leverage different assets can support. Remember that the constant assumes level payments over the full amortization schedule with no prepayment; interest-only periods, balloon structures, or floating rates will change the real annual burden, and an origination fee should always be factored into the effective constant as this calculator does.
Frequently Asked Questions
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