Future Value Calculator (9ci) Comparing Two Fixed-Rate Mortgages Who This Calculator is For: Borrowers trying to decide which of two fixed-rate mortgages they should select based on the lowest total cost over a specified future period. What This Calculator Does: This calculator compares the total cost of two fixed-rate purchase mortgages over a specified future period. It allows upfront costs to be financed,and an optional interest-only period at the beginning.