How an Adjustable Mortgage Rate Gets Calculated. There are two important terms that prospective ARM loan borrowers need to understand. When combined, these two factors determine how the adjustable mortgage rate gets calculated and applied. They are the index and the margin. The index is a general measurement of interest rates.
The initial interest rates for adjustable rate mortgages are normally lower than a fixed rate mortgage, which in turn means your monthly payment is lower. If you only plan to stay in your home for a short period of time, an ARM loan might be advantageous to you because you plan on moving or selling your home before your initial mortgage rate.
An adjustable-rate mortgage (ARM) is a 30-year mortgage where the rate is fixed for a certain time period – usually 5, 7, or 10 years. Once this fixed-rate period ends, the interest rate may adjust each year depending on the market rates at that time.