FINBIZTOOLS
Mortgage Extra Payment Calculator
See what paying a bit extra on a mortgage every month could do. Enter the loan amount, rate, original term and an extra monthly amount to compare the original schedule with an accelerated one, including the interest and time saved.
Private by design · Inputs and downloads stay in your browser
Changes the currency symbol and number format only. Amounts are not converted.
The extra amount is added to every regular payment and applied to principal, simulated month by month — not estimated by subtracting a number of months. Some lenders restrict or charge for extra payments; check your loan's actual terms.
How to use the Mortgage Extra Payment Calculator
- Enter the loan amount, annual interest rate and original loan term.
- Enter the extra amount you plan to pay every month on top of the regular payment.
- Select Calculate to compare the original and accelerated payoff side by side.
Use the currency selector above the inputs to change the symbol. It changes the display only and does not convert amounts.
How the extra payment is modeled
The extra amount is added to the regular payment every month and applied to principal, run through the exact same month-by-month engine as the original schedule — not approximated by subtracting a guessed number of months.
Because the balance falls faster, less interest accrues in every later month, which is why both the payoff time and the total interest drop together.
How the calculation works
Extra payment method
Each month: interest = balance × (annual rate ÷ 1200)
principal portion = (regular payment + extra) − interest
balance = balance − principal portion, repeated until the balance reaches zero
- The regular payment is the same fixed amount as a mortgage with no extra payment; only the extra amount changes.
With the extra payment left at 0, this produces exactly the same schedule as the original loan — the comparison isolates the effect of the extra amount alone.
Extra payment calculation example
A 320,000 loan at 6% over 30 years, with an extra 300 paid every month:
| Loan amount | 320,000 |
|---|---|
| Annual interest rate | 6% |
| Original loan term | 30 years |
| Extra monthly payment | 300 |
| Original payoff period (months) | 360 |
|---|---|
| New payoff period with extra payments (months) | 257 |
| Interest saved | 122,549.03 |
| Time saved (months) | 103 |
Amounts are shown without a currency symbol because the arithmetic is identical in any currency. The currency selector only changes the symbol and number format; it never converts amounts.
Assumptions and limitations
Assumptions
- The extra payment is a fixed amount added every month for as long as the loan remains open.
- A fixed nominal annual rate is assumed for both the original and accelerated schedules.
- No fees, insurance or rate changes are modeled.
Limitations and why results can differ
- Some lenders charge fees for extra or early payments, cap how much can be prepaid, or apply extra payments differently (for example, toward a future due date rather than principal). Check your loan's actual terms before relying on these figures.
- This does not model a one-time lump-sum extra payment, only a recurring monthly amount.
- This is an illustration, not a revised amortization statement from a lender.
Frequently asked questions
Do extra payments reduce interest?
Yes, when applied to principal. A smaller remaining balance accrues less interest every month for the rest of the loan.
Can lenders charge prepayment fees?
Some can. Prepayment penalties, caps on extra payments, and how extra amounts are applied all vary by lender — confirm your loan's terms before making extra payments.
Does the payoff time ever increase with an extra payment?
No. An extra payment applied to principal can only shorten or match the original payoff time, never lengthen it.
What if I stop paying extra partway through?
This calculator assumes the extra amount continues for the whole loan. Stopping partway would produce a result between the two schedules shown here.
Methodology
Inputs are validated: loan amount from 1 to 1,000,000,000, annual interest rate from 0% to 50%, original term from 1 to 50 whole years, and an extra monthly payment from 0 upward (0 by default).
Both the original and accelerated schedules are produced by the same shared amortization engine, called twice — once with no extra payment and once with the extra amount added to every period — so the comparison is not an estimate but two runs of the identical engine. The calculation runs in your browser and inputs are not sent to a server.
Last reviewed:
Disclaimer
Results are estimates based on the assumptions you enter. Actual repayments depend on your lender’s terms, fees and rate structure. This page is for information only and is not lending or financial advice. Read the full Disclaimer.
Explore this topic
This tool is part of Loans and Mortgages.
A common order: check what you can afford, compare loan offers, then see how extra payments change total interest.
- Mortgage Affordability Calculator
- Loan Comparison Calculator
- Mortgage Extra Payment Calculator (this page)