Mortgage Term Calculator (15b) Investing the Cash Flow Savings on a Longer Term Mortgage Who This Calculator is For: Borrowers with sufficient income to afford the higher monthly payment on a shorter term FRM, who want to know the rate of return they must earn on the cash flow savings from a longer term to make the longer term pay. What This Calculator Does: This calculator allows you to determine the rate of return on investment of the cash flow difference between a short term and a long term mortgage that would leave you as well off as if you took the short-term mortgage.