Mortgage Payoff Calculator with Extra Payments
See how extra monthly or one-time payments cut years off your mortgage and slash total interest, with a full side-by-side amortization comparison.
A mortgage payoff calculator with extra payments shows exactly how much time and interest you save by paying more than your required monthly payment. It works because a mortgage is an amortizing loan: each fixed payment is split between interest on the current balance and principal that reduces it, and any extra amount you send is applied entirely to principal. A smaller balance accrues less interest every month afterward, so the loan finishes early and the total interest falls — often by far more than the extra money you put in.
The tool above rebuilds your full amortization schedule month by month for two scenarios — your original schedule and an accelerated one with a recurring monthly extra, a one-time lump sum, or both — and reports the new payoff date, the interest saved, and a side-by-side comparison. The final payment is trimmed so the balance lands exactly on zero.
How it's calculated
The scheduled payment on a fixed-rate mortgage comes from the standard annuity formula:
Payment = P · r / (1 − (1 + r)−n)
where P is the remaining principal, r is the
monthly rate (annual rate ÷ 12), and n is the number of
payments left. Each month the simulation charges interest of
balance × r, applies the rest of the payment — plus any extra —
to principal, and carries the reduced balance forward. Because interest is
always computed on the current balance, every dollar of extra principal
removes not just that dollar but all the interest it would have generated
for the rest of the term. The exact formulas and assumptions are documented
on our methodology page, and the Consumer
Financial Protection Bureau has a plain-English explainer on
how paying down a mortgage works.
Early in a mortgage the interest share is large, which is why extra payments made early save the most. On a $320,000 balance at 6.5%, the first month's interest alone is $320,000 × (0.065 ÷ 12) = $1,733 — so of the $2,023 scheduled payment, only about $289 reduces the balance. An extra $200 that month cuts principal by nearly 70% more than the regular payment does on its own.
Worked example: $320,000 at 6.5% over 30 years
Take the calculator's default scenario: a $320,000 balance at 6.5% with 30 years remaining. The scheduled payment is $2,022.62, and following the original schedule to the end costs $408,142 in total interest — more than the amount borrowed. Here is what different extra-payment strategies do to that loan, computed with the same engine as the tool above:
| Strategy | Payoff time | Total interest | Interest saved | Time saved |
|---|---|---|---|---|
| No extra (baseline) | 30 yr 0 mo | $408,142 | — | — |
| +$100 / month | 26 yr 2 mo | $346,444 | $61,698 | 3 yr 10 mo |
| +$200 / month | 23 yr 5 mo | $302,714 | $105,429 | 6 yr 7 mo |
| +$500 / month | 18 yr 0 mo | $222,590 | $185,552 | 12 yr 0 mo |
| $10,000 one-time now | 27 yr 5 mo | $354,199 | $53,943 | 2 yr 7 mo |
| +$200 / mo and $10,000 now | 21 yr 9 mo | $269,735 | $138,407 | 8 yr 3 mo |
Read the $200 row closely: the extra totals roughly $56,000 over the shortened life of the loan, but it removes $105,429 of interest — every extra dollar eliminated almost two dollars of cost. The one-time $10,000 row shows the power of timing: a single payment today saves $53,943 because that principal stops generating interest for the remaining decades of the loan. Your own numbers will differ with your balance, rate, and remaining term; enter them above to get your exact schedule.
When should you use this calculator?
Use it whenever you want to see the mechanical effect of a specific extra payment before you commit the money. Common cases: checking what a windfall — a bonus or tax refund — does to your payoff date if applied as a lump sum; comparing a recurring $100 versus $200 monthly extra; modeling one extra payment a year by entering one-twelfth of your payment as a monthly extra; or verifying the savings a bi-weekly plan promises before signing up for one. It is also useful after the fact, to confirm your servicer applied an extra payment to principal — the new balance should match the schedule the tool builds.
What this calculator deliberately does not answer is whether prepaying your mortgage is the best use of the money compared with investing, holding cash, or paying other debt. That depends on your rate, your alternatives, your tax situation, and your risk tolerance. The tool gives you the exact cost side of that decision; the judgment is yours.
Assumptions and limitations
- The interest rate is fixed for the remaining term; adjustable-rate loans are not modeled.
- Extra amounts are applied to principal in the same month they are paid. Confirm your servicer does the same — see the FAQ below.
- Escrow items (property taxes, homeowners insurance, PMI) are excluded; the payment here is principal and interest only.
- No prepayment penalty is modeled. Some mortgages charge one; the CFPB explains what a prepayment penalty is and where to find it in your documents.
- Recasting (re-amortizing to a lower payment after a lump sum) is not modeled; the tool assumes your payment stays the same and the term shortens.
- Results are estimates for planning. Your servicer's day-count and rounding conventions can shift figures slightly.
This page is educational content about how mortgage amortization responds to extra payments. It is not financial advice, and it does not consider your personal circumstances. For decisions that matter, consult a qualified professional.
Last reviewed: 2026-07-09