Most first-time homebuyers are a little stretched financially, so they want payments as low as possible at the beginning. With FHA's adjustable rate mortgage (ARM), the initial interest rate and monthly payments are low, but these may change during the life of the loan. FHA uses the 1-Year Constant Maturity Treasury Index (1 Yr CMT the most widely used index, to calculate the changes in interest rates. An index is a measure of interest rate changes that determine how much the interest rate on an ARM will change over time.
The maximum amount that the interest rate on your loan may increase or decrease in any one year is 1 or 2 percentage points, depending upon the type of ARM you choose. Over the life of the loan, the maximum interest rate change is 5 or 6 percentage points from the initial rate, again depending upon the type of ARM you choose. The advantage of an ARM is that you may be able to afford more house; because your initial interest rate will be lower, as will your payment.