How extra mortgage payments affect payoff time and interest
This calculator estimates the time needed to pay off your remaining principal at a constant monthly interest rate and payment. It compares the regular payment alone with the regular payment plus any extra monthly amount you enter, and reports the modeled interest difference.
How the estimate works
Each month, estimated interest is calculated from the remaining balance and the monthly rate (annual rate ÷ 12 ÷ 100). The scheduled payment covers that month’s interest first; the rest reduces principal. An extra payment can reduce the balance sooner, so less interest may accrue in later months—provided the lender applies the extra amount to principal.
How to compare scenarios
Enter the current balance, annual rate, and your regular principal-and-interest payment. With extra payment set to $0, note the baseline payoff time and interest estimate; then enter an extra monthly amount to compare. The comparison is an estimate, not a promise of exact savings.
The model assumes a fixed rate, regular monthly payments, and extra payments applied to principal. It does not include escrow, fees, payment-date differences, or all contract-specific servicing rules. Before changing your payment, ask your servicer how extra principal payments are handled and check your loan terms. The Consumer Financial Protection Bureau explains principal, interest, and mortgage amortization.
Tips for accurate results
Enter your current balance, interest rate, remaining term, and the extra amount you plan to add each month. The comparison shows how many months you could cut and how much interest you could avoid. Ask your servicer to confirm that extra payments go to principal.
Common mistakes and limits
Before paying extra, compare the guaranteed return of saving mortgage interest with other priorities, such as higher-interest debt, an emergency fund, or retirement accounts. Check for prepayment penalties, and confirm the exact payoff amount with your servicer because interest accrues daily.
Learn more: How mortgage payments are calculated.