In adjustable rate mortgages (ARM's), the interest rate changes periodically by adding what’s referred to as a “margin” to an index specified in mortgage documents. These two numbers are combined to create the loan’s interest rate and often times have limits (sometimes referred to as “caps and collars”) that ensure the rate does not increase over a certain amount over the life of the loan. As an example, a 1-year ARM will adjust every year, typically on the anniversary date of the loan.
Because the rate changes as the index changes with fluctuations in the market, monthly payments on an adjustable rate mortgage loan likely will be different every year. However, if you are planning on being in your home a short period of time, an ARM may be a very good option with a lower interest rate.