Future Value Calculator (9bi) Comparing a Negative Amortization ARM with a FRM Who This Calculator is For: Borrowers trying to decide whether they should select an adjustable rate mortgage (ARM) that allows 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 negative amortization ARM with that of a fixed-rate mortgage, over a specified future period. It allows the upfront refinance costs to be financed.