Future Value Calculator (9ai) Comparing an ARM with a FRM Who This Calculator is For: Borrowers trying to decide whether they should select an adjustable rate mortgage (ARM) without negative amortization or a fixed rate mortgage (FRM) based on the lowest cost over a specified future period. What This Calculator Does: This calculator compares the total cost of a new adjustable rate mortgage with that of a new fixed-rate mortgage, over a specified future period.