What this calculator does
This mortgage payoff calculator simulates your loan month by month. Enter the loan amount, the annual interest rate, the original term and any extra amount you pay toward the principal each month. It then shows your payoff date, how long the loan really lasts, the total interest you will pay, and how much interest the extra payments save compared with making only the minimum payment.
How the math works
Your regular monthly payment comes from the standard amortization formula:
- Monthly payment = P × i ÷ (1 − (1 + i)−n), where P is the loan amount, i is the monthly rate (APR ÷ 12) and n is the number of months.
- Each month, the interest charge is the remaining balance × i, and everything above that reduces the principal.
- An extra payment goes straight to principal, which shrinks the next month's interest charge. The effect compounds: every extra dollar kills future interest on itself.
Worked example
A $400,000 loan at 6.5% over 30 years has a standard payment of about $2,528 a month and costs about $510,000 in total interest. Add an extra $300 a month and the loan is paid off in about 269 months (roughly 22.5 years), with total interest of about $361,000. That is roughly $149,600 less interest and about 7.5 years off the loan, from $300 a month.
Why extra payments are so powerful
In the early years of a 30-year loan, most of your payment is interest. Cutting the principal early removes interest charges from every remaining month at once. That is why even a modest extra payment, started early and kept up, can shave years off a mortgage. Lump sums (a bonus, a tax refund) work the same way: applied to principal, they skip you forward on the amortization schedule.
Limitations
This is an estimate. It assumes a fixed rate, no prepayment penalty, no escrow changes, and that every extra dollar goes to principal (confirm with your servicer that it is applied that way). It does not model refinancing, adjustable rates, or the opportunity cost of investing the extra money instead. Always check your loan documents for prepayment terms.