Use the Option ARM Calculator to compare a fixed rate mortgage to an Option ARM. A fixed rate mortgage has the same payment for the entire term of the loan. The Option ARM uses a low initial rate to calculate your initial minimum monthly payment. Although the interest rate will increase after 1 to 3 months, your low payment will remain fixed for the entire year. This can produce a much lower monthly payment than a traditional fixed rate mortgage, or even an adjustable rate mortgage (ARM).