What is a Mortgage Constant and How is it Calculated?

What is a Mortgage Constant?

A mortgage constant, also known as a loan constant, is a metric used in real estate finance to measure the annual debt service (the total principal and interest payments for the year) relative to the total loan amount. It is expressed as a percentage and provides a quick way to determine the percentage of a loan amount that would need to be paid annually to cover both principal and interest payments.

How is a Mortgage Constant Calculated?

The mortgage constant is calculated using the following formula:

Mortgage Constant = Annual Debt Service / Total Loan Amount
  • Annual Debt Service is the total amount paid in a year for both principal and interest.
  • Total Loan Amount is the original amount borrowed.

To find the Annual Debt Service, you can use the formula for the annuity payment, which can be derived from:


  • P is the loan principal (or total loan amount).
  • r is the annual interest rate (expressed as a decimal).
  • n is the total number of payments (years multiplied by the number of payments per year).

This formula allows the calculation of the constant based on a fixed-rate mortgage where payments (covering both principal and interest) are the same throughout the life of the loan. The mortgage constant helps investors and lenders quickly assess the cash flow implications of a loan, compare the cost-effectiveness of different loan options, and determine the income needed from a property to cover its debt obligations.

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