The yearly percentage of interest which remains the same over the life of an amortized loan, based on the monthly payment in relation to the principal originally loaned. For example: A $1000 loan at 9% interest for 20 years can be amortized at $9.00 per month. A constant interest rate is figured by finding one year’s payments ($9.00 X 12 months = $108.00), and expressing this amount as a percentage of the principal originally borrowed (10.8% of $1000).