The adjustment interval is the period of time between allowable interest rate changes on an adjustable rate mortgage (ARM). ARMs come with two adjustment intervals. The first specifies the length of time that the interest rate is initially fixed. The second specifies how often an interest rate can adjust, after the initial fixed-rate period on the mortgage ends. For example, a 5/1 ARM has an initial period where the rate is fixed for 5 years, adjusts at the end of the fixed 5 year period, and then can adjust every 1 year thereafter.