The direct answer
Loan constant is annual debt service divided by the original loan amount, usually expressed as a percentage. It summarizes the annual payment burden of a loan structure and can help compare financing scenarios, but it is not the interest rate, debt yield or DSCR.
Calculate it from the actual payment structure
Use annual principal-and-interest debt service for the modeled loan and divide by the original principal. Amortization term, note rate and payment frequency all affect the constant. Interest-only periods require separate treatment because the payment structure changes.
To apply the approach to an editable scenario, use the Debt Yield Calculator. For the neighboring question, explore Commercial Real Estate Investment Calculator.
Use it as a bridge between loan amount and debt service
Multiplying a proposed loan amount by an applicable annual loan constant gives an estimate of annual debt service for that structure. This can support quick sizing and scenario analysis before a full amortization schedule is reviewed.
Keep it separate from property performance ratios
Debt yield relates NOI to loan amount, while DSCR relates NOI to debt service. Loan constant describes the loan payment structure itself. Comparing them can be useful, but they are not interchangeable.
Worked example
If a $5,000,000 loan requires $390,000 of annual principal-and-interest payments, the annual loan constant is 7.8%. A $4,000,000 loan with the same structure would imply about $312,000 of annual debt service before considering fees or structural differences.
Common mistakes and limitations
Do not call the loan constant the mortgage rate. Amortization causes the annual payment burden to differ from the note rate.
Debt yield is not DSCR or LTV and does not incorporate interest rate or amortization.
Frequently asked questions
Is loan constant the same as interest rate?
No. The constant reflects scheduled debt service relative to original principal and therefore incorporates amortization.
Can I use loan constant to estimate debt service?
Yes, for the same loan structure, as a screening calculation; use the actual amortization schedule for final underwriting.
Sources and further reading
The article’s examples are illustrative arithmetic. The references below provide context for operating, valuation or transaction concepts; they do not endorse the assumptions or the calculator.
- Office of the Comptroller of the Currency — Comptroller's Handbook — Commercial Real Estate LendingNOI, direct capitalization, cap-rate valuation and DCF concepts for commercial real estate.
Read our editorial policy for the scope of these educational methods.