Because the interest rate is not locked in, the monthly payment for this type of loan will change over the life of the loan. Most ARMs have a limit or cap on how much the interest rate may fluctuate, as well as how often it can be changed. When the rate goes up or down, the lender recalculates your monthly payment so that you’ll make equal payments until the next rate adjustment occurs.

As interest rates rise, so does your monthly payment, with each payment applied to interest and principal in the same manner as a fixed-rate mortgage, over a set number of years. Lenders often offer lower interest rates for the first few years of an ARM, but then rates change frequently after that – as often as once a year. The initial interest rate on an ARM is significantly lower than a fixed-rate mortgage.