Mortgage Prepayment Calculator (2d) Property Appreciation and Extra Monthly Payments on Underwater Mortgages Who This Calculator is For: Borrowers with negative equity (they are "underwater") who want to know, how long it will take before some combination of normal amortization, extra payments and, property appreciation eliminate the negative equity. What This Calculator Does:This calculator provides amortization schedules and homeowner equity for any combination of extra payments and property appreciation.