How extra mortgage payment calculator works
This calculator compares your mortgage's standard amortization schedule with an accelerated one where you pay extra toward principal each month, optionally plus a one-time lump-sum principal payment. Enter your current balance, interest rate, and remaining years (or your required monthly payment), then add any extra monthly amount and lump sum. The tool produces a year-by-year schedule showing the remaining balance and interest paid each year under both scenarios, plus your payoff date, years shaved off, and total interest saved.
Each month the calculator accrues interest on the remaining balance at the monthly rate (annual rate divided by 12), then applies your scheduled payment plus the extra to that interest first and the remainder to principal. A lump sum is applied directly to principal at the start, before the first month's interest accrues, so every subsequent month's interest is computed on a smaller base. Adding even a small extra each month compounds: the balance drops faster, so less interest accrues every following month, which is where the real savings come from.
Payoff dates are computed from a start date (today by default, or any date you choose) by adding the number of months to payoff. This is a planning estimate for a fixed-rate mortgage with level payments; it does not model escrow, taxes, insurance, rate changes, or prepayment penalties. Some lenders impose limits or fees on extra principal payments, so confirm the terms with yours. This tool is for education and is not financial advice.