Loan Constant Calculator
Enter the loan amount, interest rate, and amortization period to estimate the annual debt service as a share of the original principal.
Loan constant7.58%
- Monthly payment
- $6,320.68
- Annual debt service
- $75,848.16
- Total interest
- $1,275,444.88
- Number of payments
- 360
Assumes a fixed interest rate, equal monthly payments, and full amortization over the stated period.
Excludes fees, taxes, insurance, reserves, balloon payments, and interest-only periods.
How to use this calculator
- Enter the original loan amount before fees or reserves.
- Enter the fixed annual interest rate used for principal-and-interest payments.
- Enter the amortization period in years.
- Read the loan constant beside the monthly payment, annual debt service, and total interest.
How the loan constant is calculated
The loan constant is the annual principal-and-interest debt service divided by the original loan amount. It turns a fully amortizing loan into a single yearly percentage, which makes it useful when comparing debt service to property income or sizing a commercial real estate loan.
This calculator first solves for the fixed monthly payment, then annualizes that payment. Let L be the loan amount, R be the annual interest rate as a decimal, Y be the amortization period in years, i = R / 12 be the monthly rate, n = 12Y be the number of monthly payments, and M be the monthly payment.
When the rate is greater than zero, the monthly payment is:
M = L[i(1 + i)^n] / [(1 + i)^n - 1]
When the rate is zero, the payment is simply:
M = L / n
Annual debt service is D = 12M. The loan constant is C = (D / L) x 100%. Total interest is I = nM - L.
What moves the result
The interest rate and the amortization period do most of the work. A higher rate raises the payment and the constant. A shorter amortization period also raises the constant because more principal has to be repaid each year. For a positive-rate amortizing loan, the constant is higher than the interest rate because the annual payment includes both interest and scheduled principal.
The original loan amount changes the dollar payment and annual debt service, but it does not change the constant when the rate and amortization period stay the same. A $1,000,000 loan and a $10,000,000 loan with the same rate and amortization schedule have the same loan constant.
What this leaves out
This is a fixed-rate, fully amortizing calculation. It does not model lender fees, points, taxes, insurance, reserves, prepayment penalties, day-count conventions, balloon maturities, interest-only periods, or rate changes. Those items can affect the actual cash requirement even when the stated note rate and amortization period look the same.
Worked example
Suppose a loan has a $1,000,000 original balance, a 6.5% fixed annual interest rate, and a 30-year amortization period. The monthly rate is 6.5% / 12, and the schedule has 360 monthly payments. The fixed monthly principal-and-interest payment is $6,320.68.
Annual debt service is $6,320.68 x 12, or $75,848.16. Dividing that amount by the $1,000,000 original principal gives a loan constant of 7.5848%. Over the full amortization period, scheduled payments total $2,275,444.88, so total interest is $1,275,444.88.
Common questions
What is a loan constant?
A loan constant is annual debt service divided by the original loan amount. It is usually shown as a percentage, so a 7.50% constant means scheduled principal-and-interest payments equal 7.50% of the original principal each year.
How is a mortgage constant calculated?
A mortgage constant is calculated by finding the fixed monthly payment, multiplying it by 12, and dividing that annual debt service by the original loan amount. The result depends on the interest rate and amortization period.
Is the loan constant the same as the interest rate?
No. The interest rate is the cost of borrowing before scheduled principal repayment. The loan constant includes both interest and principal, so it is higher than the interest rate for a positive-rate fully amortizing loan.
Why is an amortizing loan's constant higher than its rate?
Each payment covers interest and also pays down principal. Because the loan constant measures the full annual payment against the original principal, it includes that principal repayment as well as interest.
How do lenders use the loan constant in commercial real estate?
Lenders use the constant to translate a loan amount into annual debt service. That annual debt service can then be compared with net operating income when estimating debt service coverage and loan sizing.
What is the loan constant for an interest-only loan?
For a pure interest-only loan with no principal repayment, the constant equals the interest rate. This calculator does not model interest-only periods because it assumes the loan fully amortizes through equal monthly payments.