Use our adjustable rate mortgage calculator to determine the total amount you will pay over the course of your loan. Adjustable rate mortgages involve a trade-off. Initially, the borrower gets a lower interest rate, but must accept the risk that interest rates might rise in the future. However, if the interest rates decline, the borrower stands to benefit. The ARM loans are usually repaid over a 30 year period, but monthly payments may increase or decrease over that period of time, depending on the movement of interest rates.