Part of our guide to How Much House Can You Actually Afford?
An extra payment behaves differently from a normal one, and the difference is why modest overpayments produce results people assume are a mistake.
The interest you never pay and the time you cut off the term by adding a fixed amount each month. The saving is far larger than the amount added, because every extra dollar removes both the principal and all the future interest that principal would have accrued.
Why the saving is so much larger than the payment
A scheduled mortgage payment is split: part covers the interest accrued that month, the remainder reduces the balance. In the early years that split is heavily weighted toward interest.
An extra payment has no interest to cover. It is applied entirely to principal. And principal removed today never generates interest again — not next month, not in year 19.
So adding $200 a month does not save you $200 a month. It removes a slice of principal and every interest charge that slice would have produced across the remaining term. That is why the totals look implausible until you see the arithmetic.
How the calculation works
Two full amortisation schedules, simulated month by month:
interest = balance × (rate / 12)
balance = balance + interest − paymentOnce with the scheduled payment, once with the payment plus your extra. The difference in months and in accumulated interest is the result.
Simulating rather than using a closed form matters here: the term shortens as you overpay, so there is no fixed number of periods to plug into a formula.
How to read the result
Check the years, not just the money. Cutting a 30-year term to 24 changes when you own the house outright, which is a different kind of outcome from a number on a screen.
Compare against your other debts first. Overpaying a 6% mortgage while carrying a 22% card balance is the wrong order — see how to pay off debt faster.
Keep the emergency fund. Money put into a mortgage is difficult to get back out. A buyer with no reserve meets the first repair bill on a credit card, which undoes the arithmetic.
Important considerations
Tell your lender the extra is for principal. Some apply unlabelled extra money to the next scheduled payment instead, which saves you nothing at all. Verify on the following statement.
Recasting is not the same thing. Some lenders will re-amortise after a lump sum, lowering the payment while keeping the term. That is the opposite trade to the one modelled here.
This assumes a fixed rate. On an adjustable-rate mortgage the payment moves, so treat the output as indicative — see fixed vs adjustable-rate mortgages.
Related tools and reading
- Mortgage payment calculator
- Home affordability calculator
- How to pay off debt faster
- Fixed vs adjustable-rate mortgage
This is an estimate for general information, not financial advice — see our disclaimer.
Frequently asked questions
Why does a small extra payment save so much interest?
Because it goes entirely to principal. Removing principal also removes every future interest charge that principal would have generated, so the saving compounds over the remaining term.
Is paying extra better than investing the money?
Paying down a mortgage is a guaranteed, risk-free return equal to your interest rate. An investment might beat it and might not. The higher your rate, the stronger the case for overpaying.
Should I clear higher-interest debt first?
Almost always yes. A credit card at 22% costs far more than a mortgage at 6%, so the same money does more work there.
Will my lender apply the extra to principal automatically?
Not always. Some hold it as a prepayment of the next instalment, which saves nothing. Tell them explicitly it is a principal reduction, and check the next statement.
Are there prepayment penalties?
Uncommon on US mortgages now but not extinct, and more likely on non-standard loans. Check the note before committing to a schedule.
Sources
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