Mortgage overpayment calculator
See what paying a little more each month does to the finish date and the total interest. The effect is usually much larger than people expect.
How this is calculated
The normal monthly payment is worked out from the balance, rate and remaining term. The overpayment is added on top and the whole thing is simulated month by month until the balance clears.
Every extra pound goes straight against the capital, which removes all the future interest that pound would have attracted. That is why a modest overpayment early in a term has such a large effect.
The interest saved is the difference between the total interest paid with and without the overpayment.
The calculation assumes the rate stays the same throughout, which it will not. Treat the result as the shape of the saving rather than a precise forecast.
Rates used · 2026/27
| Typical penalty-free limit | 10% of the balance a year |
| Early repayment charge | commonly 1% to 5% while in a fixed deal |
| Effect of overpaying early | much larger than overpaying late |
| Rate assumed | fixed for the whole term |
Last checked July 2026. Source: HMRC and gov.uk.
Worked example
On £240,000 at 4.5% over 30 years the normal payment is £1,216.04 and the interest totals £197,776. Overpaying £200 a month clears the mortgage in 22 years 6 months instead of 30, saving £55,889 of interest. The £200 buys back seven and a half years.
Common questions
Am I allowed to overpay?
Usually up to 10% of the balance a year without penalty while in a fixed deal. Beyond that an early repayment charge normally applies, so check your agreement first.
Should I overpay or save instead?
Compare your mortgage rate against the savings rate you could get after tax. If the mortgage rate is higher, overpaying wins, and it is a guaranteed return rather than a hoped-for one.
Should I reduce the term or the payment?
Reducing the term saves more interest. Reducing the payment gives you flexibility. Overpaying voluntarily gives you both, since you can stop any month.
Does overpaying affect my credit file?
No. Clearing a mortgage early is neutral to positive, and the account simply closes when the balance reaches zero.