With an adjustable-rate mortgage, the initial interest rate is fixed for a period of time generally 1, 3, 5, 7 or 10 years depending on the loan you select. After this initial period of time, the interest rate can change periodically, at yearly intervals or in some cases every 5 years. The interest rate for an ARM is reset based on a benchmark or index, plus an additional spread called a margin. Your rate will be determined by adding the margin and the index. ARMs typically have lower starting rates which can help lower your payments or allow you to qualify for a higher loan amount.