Mortgage Payoff Calculator — Extra Payments & Early Payoff
See how extra monthly payments pay off your mortgage early: interest saved, years saved and a full accelerated amortization schedule, computed to the cent.
A clear path from inputs to results.
See what extra monthly prepayments do to your mortgage: how many years earlier you finish, how much interest you avoid, and the month-by-month schedule that gets you there. The tool runs your loan twice — once at the scheduled payment and once with your extra amount — and plots both balance curves side by side.
How to use
- Enter your current principal balance — not the original loan amount.
- Enter the interest rate and the years remaining on the loan. The tool derives the scheduled principal-and-interest payment from these three numbers.
- Set the extra monthly prepayment (the slider runs from $0 to $5,000). Even $0 is informative: it shows your baseline interest cost.
- Read the headline: total interest saved by prepaying, followed by years saved, the scheduled payment, the baseline interest and the number of months to payoff.
- Compare the two balance curves — accelerated versus standard — and open the accelerated amortization ledger for the month-by-month detail.
The formula
Reference: Annuity-immediate payment (Broverman, Mathematics of Investment and Credit); the accelerated path is simulated month by month with 20-significant-digit decimal arithmetic (Decimal.js), capping the final payment at the remaining balance.
Worked example
$400,000 balance, 6.5% APR, 30 years remaining, $500 extra
- Balance $400,000; rate 6.5%; 30 years remaining (360 months); extra monthly prepayment $500.
- 1
The scheduled payment on $400,000 at 6.5% over 360 months is $2,528.27, so prepaying lifts the real outlay to $3,028.27 a month. Paying only the scheduled amount accrues $510,177.95 of interest over 360 months. With the extra $500 applied every month, interest is recomputed on a smaller balance each month and the loan retires in month 233 — 127 months (10.6 years) early — with total interest of $304,620.81. The difference, $205,557.14, is the interest saved: about 40% of the baseline interest bill.
Payoff in 233 months (19 years 5 months) instead of 360; interest $304,620.81 instead of $510,177.95; $205,557.14 saved over 10.6 years.
Interpreting the result
The interest-saved figure is money you never owe, but it arrives as a shorter loan rather than a smaller payment — your required payment stays at $2,528.27 and the extra $500 simply buys principal. The chart shows the accelerated curve falling faster from month one, with the gap widest in the middle years. If cash is tight, smaller extras still compound: $200 a month saves $111,891.54 on this same loan.
Assumptions & limitations
- The rate stays fixed and interest accrues monthly on the outstanding balance; taxes, insurance and escrow are not modeled (principal and interest only).
- The extra payment applies every month from month one until the balance is gone; no lump sums or skipped months.
- The final payment is capped at the remaining balance plus interest, so the accelerated schedule ends cleanly at zero.
- Months saved are measured against the full remaining term you enter, and no prepayment penalties are modeled.
Frequently asked questions
How much will an extra $200 a month save me?
On a $400,000 balance at 6.5% with 30 years left, $200 a month cuts the payoff from 360 to 293 months — 5.6 years early — and saves $111,891.54 of interest. The extra cash contributed over those 293 months is about $58,600, so the avoided interest comes to roughly $1.90 per extra dollar because each prepaid dollar stops accruing for years.
Will my monthly payment go down (recast)?
No. The model keeps your required payment at the scheduled level and applies the extra entirely to principal, so the benefit arrives as an earlier payoff date rather than a lower payment. A lower required payment requires a formal recast with your servicer, which is a different transaction not modeled here.
What happens if I enter 0 extra?
The accelerated path becomes the baseline: the tool shows the scheduled payment, the total interest over the remaining term and zero savings. That is a useful starting point — the baseline interest figure is the number every extra-payment scenario is measured against.
Does the calculator use my original loan amount?
No. It starts from your current balance and the years remaining, which is what you actually owe today. Entering the original amount and full term reproduces a brand-new loan instead — useful for comparison, but the accelerated scenario is most accurate from your real current balance.
Why is the interest saved so large?
Interest is charged on the balance every month, so each prepaid dollar shrinks the base on which all future interest is calculated. In the worked example, avoiding $205,557.14 of interest costs about $116,500 of extra $500 payments before payoff — and the remaining gap comes from the 127 scheduled payments that never have to be made because the loan ends early.
Are the results exact?
The worker computes with 20-significant-digit decimal arithmetic (Decimal.js) and reports to the cent, using the same monthly recursion a servicer applies: interest = balance × APR ÷ 12, then payment minus interest reduces principal. Your actual payoff can differ by a few dollars if your lender uses actual/365 day counts or payments have been late.
Methodology last reviewed 2026-10-06. Results are informational and not financial or engineering advice — see our disclaimer.