Extra Mortgage Payment Calculator
See exactly how much interest an extra monthly payment saves, and how many years it takes off your mortgage.
Schedule with your extra payment
| Month | Payment | Interest | Principal | Balance |
|---|---|---|---|---|
| 1 | $2,412.24 | $1,895.83 | $516.40 | $349,483.60 |
| 2 | $2,412.24 | $1,893.04 | $519.20 | $348,964.39 |
| 3 | $2,412.24 | $1,890.22 | $522.01 | $348,442.38 |
| 4 | $2,412.24 | $1,887.40 | $524.84 | $347,917.54 |
| 5 | $2,412.24 | $1,884.55 | $527.68 | $347,389.85 |
| 6 | $2,412.24 | $1,881.70 | $530.54 | $346,859.31 |
| 7 | $2,412.24 | $1,878.82 | $533.42 | $346,325.89 |
| 8 | $2,412.24 | $1,875.93 | $536.31 | $345,789.59 |
| 9 | $2,412.24 | $1,873.03 | $539.21 | $345,250.38 |
| 10 | $2,412.24 | $1,870.11 | $542.13 | $344,708.24 |
| 11 | $2,412.24 | $1,867.17 | $545.07 | $344,163.17 |
| 12 | $2,412.24 | $1,864.22 | $548.02 | $343,615.15 |
Overpayments go straight to principal, so the interest saved compounds. The same extra payment made five years from now saves substantially less.
How this is calculated
Two schedules are built — one at the contractual payment, one with your extra payment applied to principal each month — and compared on total interest and payoff date.
An extra payment on an amortizing loan behaves very differently from an extra payment on anything else you buy, because it does not just reduce your debt by its own size — it removes every future interest charge that debt would have generated.
When you pay $200 above your required payment, the lender takes the month's interest from the scheduled amount as normal, and the entire $200 comes off the principal. Next month, interest is charged on a balance that is $200 smaller. That saving is tiny in isolation, but it repeats every month for the remaining term, and each month's saving slightly reduces the balance again. The effect compounds in your favour, which is the mirror image of how the loan compounds against you.
This is why the timing of an extra payment matters enormously. A dollar paid in year one of a 30-year mortgage avoids 29 years of interest on that dollar. The same dollar paid in year 28 avoids two. On a typical 30-year loan at 6.5%, $200 a month from the start removes roughly seven years and a six-figure sum in interest; starting the same $200 a decade in saves a fraction of that.
Two practical cautions. First, make sure the overpayment is actually applied to principal — some servicers will otherwise treat it as an advance on next month's payment, which achieves almost nothing. Say so explicitly when you pay, and check the next statement shows a lower balance rather than a credit.
Second, this is not automatically the best use of the money. Overpaying a mortgage is a guaranteed, tax-free return equal to your interest rate, and it is also illiquid — that money is very hard to get back without selling or refinancing. Clearing higher-rate debt first, and holding a cash reserve, both usually come before overpaying a mortgage.
Worked examples
$200 extra from the start
A new $350,000 mortgage at 6.5% over 30 years, with $200 a month extra from day one.
About seven years come off the term and the interest saving runs well into six figures — several times the roughly $55,000 of extra payments you actually make.
Load this scenario into the calculatorOne extra payment a year, spread monthly
The same loan, with the monthly payment divided by twelve added each month — roughly $184 extra.
This is the 'thirteenth payment' strategy. It produces almost exactly the same result as a biweekly plan, without needing your lender to support one or paying a service to set it up.
Load this scenario into the calculatorStarting ten years in
A $290,000 balance with 20 years left at 6.5%, adding $200 a month.
The same $200 saves substantially less than it would have on day one, because there are fewer years of future interest left to cancel. Overpaying is worth doing at any point, but it is worth far more early.
Load this scenario into the calculatorWhat this assumes
- Extra payments are applied to principal immediately, not held as a credit against the next scheduled payment.
- The rate is fixed and the extra payment is made every month without interruption.
- No prepayment penalty is applied. These are rare on US residential mortgages but common on some UK fixed-rate deals during the fixed period.
- The required monthly payment is derived from the balance, rate, and remaining term, so it may differ slightly from your actual payment if you have already overpaid or if fees are bundled in.
- Principal and interest only — escrow items such as taxes and insurance are excluded.
Common questions
- Is it better to make one big extra payment or add a little each month?
- Earlier is better, so a lump sum now beats the same total spread over a year. But the difference is small compared to the difference between overpaying and not overpaying at all. Whichever you will actually keep doing is the right choice.
- Will my monthly payment go down if I overpay?
- Usually not. Most lenders keep the payment the same and shorten the term instead, which is where the interest saving comes from. Some will 'recast' the loan on request after a large lump sum, lowering the payment over the original term — that reduces your monthly outgoing but saves far less interest.
- Should I pay off my mortgage early or invest the money?
- Compare your mortgage rate against your realistic after-tax investment return. Overpaying gives a guaranteed return equal to your rate with no risk; investing offers a higher expected return with real risk and better liquidity. At 3% the case for investing is strong; at 7% overpaying is hard to beat on a risk-adjusted basis.
- How do I make sure the extra goes to principal?
- Tell your servicer explicitly, in writing where possible, that additional funds are for principal reduction. Then check the following statement: the balance should have dropped by your extra payment. If it shows as a credit or 'paid ahead', the payment is not doing what you intended.
- Are there penalties for paying a mortgage off early?
- In the US, prepayment penalties on residential mortgages are heavily restricted and uncommon on modern loans, though not impossible. In the UK, early repayment charges during a fixed-rate period are standard and can be several percent of the balance. Check your agreement before making large overpayments.
Related calculators
- Amortization Schedule Calculator — See the full month-by-month breakdown of any loan — what you pay, what goes to interest, and what you still owe.
- Biweekly Mortgage Payment Calculator — Compare paying half your mortgage every two weeks against paying monthly, and see what the switch actually saves.
- Compound Interest Calculator — Project savings or investments year by year, separating what you contribute from what compounding adds.