Interest Cost Calculator (9b) Fixed-Rate Mortgage Versus ARM With Negative Amortization Who This Calculator is For: Borrowers trying to decide whether they should select an adjustable rate mortgage (ARM) with negative amortization or a fixed rate mortgage (FRM) based on the lowest after-tax interest cost. What This Calculator Does:This calculator compares the interest cost on an ARM with negative amortization and an FRM, taking account of all financing costs, before and after taxes, over expected holding periods chosen by the user.