Mortgage Prepayment Calculator (2b) Biweekly Payments Applied Monthly Who This Calculator is For: Borrowers who want to know when their loan will pay off, and how much interest they will save, if they shift to a biweekly payment plan, and if they make extra voluntary payments in addition to their required biweekly payment. What This Calculator Does:This calculator provides amortization schedules for biweekly payment mortgages, with or without additional payments, where the payments are applied monthly.