UK Mortgage Overpayment Calculator
Find out how much interest a mortgage overpayment saves and how many years it cuts from your term. Enter your outstanding balance, interest rate and the years remaining, then add a regular monthly overpayment, a one-off lump sum, or both to compare against your current schedule.
| Year | Balance without overpaying | Balance with overpaying | Difference |
|---|---|---|---|
| 1 | £195,569.38 | £193,119.26 | £2,450.12 |
| 2 | £190,935.22 | £185,922.41 | £5,012.81 |
| 3 | £186,088.16 | £178,394.95 | £7,693.22 |
| 4 | £181,018.44 | £170,521.67 | £10,496.77 |
| 5 | £175,715.81 | £162,286.70 | £13,429.11 |
| 6 | £170,169.58 | £153,673.41 | £16,496.17 |
| 7 | £164,368.55 | £144,664.43 | £19,704.12 |
| 8 | £158,301.03 | £135,241.58 | £23,059.45 |
| 9 | £151,954.76 | £125,385.85 | £26,568.92 |
| 10 | £145,316.96 | £115,077.34 | £30,239.61 |
| 11 | £138,374.21 | £104,295.26 | £34,078.94 |
| 12 | £131,112.51 | £93,017.86 | £38,094.65 |
| 13 | £123,517.21 | £81,222.38 | £42,294.83 |
| 14 | £115,572.98 | £68,885.01 | £46,687.97 |
| 15 | £107,263.80 | £55,980.87 | £51,282.93 |
| 16 | £98,572.90 | £42,483.91 | £56,088.99 |
| 17 | £89,482.73 | £28,366.90 | £61,115.83 |
| 18 | £79,974.97 | £13,601.36 | £66,373.60 |
| 19 | £70,030.42 | £0.00 | £70,030.42 |
| 20 | £59,629.02 | £0.00 | £59,629.02 |
| 21 | £48,749.78 | £0.00 | £48,749.78 |
| 22 | £37,370.75 | £0.00 | £37,370.75 |
| 23 | £25,468.97 | £0.00 | £25,468.97 |
| 24 | £13,020.43 | £0.00 | £13,020.43 |
| 25 | £0.00 | £0.00 | £0.00 |
How it works
A repayment mortgage charges interest on the balance you still owe. Every pound of overpayment goes straight to that balance, so it removes not only that pound of debt but all the interest it would have attracted for the rest of the term. That is why small, early overpayments matter more than large, late ones.
What the calculator does
It builds two amortisation schedules month by month at the rate you enter. The first uses your normal payment for the full remaining term. The second applies your lump sum immediately, then adds your monthly overpayment to every payment and keeps paying at that level until the balance reaches zero. Comparing the two gives the interest saved and the months removed.
Shortening the term versus cutting the payment
When you overpay, a lender can either keep your payment the same and end the mortgage sooner, or recalculate a lower payment over the original term. The first saves far more interest, and it is what this calculator models. If you want the second, ask your lender explicitly.
Early repayment charges
Fixed-rate deals commonly allow 10% of the balance to be overpaid each year penalty-free, with a charge of 1% to 5% on anything above that. Check your mortgage offer, and if your allowance is tight consider saving the surplus and paying a lump sum when the fixed period ends.
Overpay or save?
Overpaying gives a guaranteed return equal to your mortgage rate, and it is tax free. Compare it with the after-tax return on savings, and keep three to six months of expenses accessible first. To see how the base payment is derived, use the mortgage repayment calculator.
Worked example
£200,000 balance, 4.5% interest, 25 years remaining. The normal monthly payment is about £1,111.66 and total interest over the full term comes to roughly £133,498.
- Add £200 a month and the mortgage clears in about 21 years instead of 25.
- Total interest falls to roughly £109,000, saving around £24,000 of interest.
- A £10,000 lump sum today instead would save a similar amount of interest and take about 2 years off the term.
Change the rate and the saving moves with it — higher rates reward overpayment more.
Limitations of this calculator
- Assumes one fixed interest rate for the whole remaining term.
- Assumes a capital-and-interest (repayment) mortgage, not interest-only.
- Ignores early repayment charges, product fees and annual overpayment limits.
- Assumes overpayments are credited immediately; some lenders only apply them at an annual review.
- Excludes buildings insurance, ground rent, service charges and mortgage protection.
- An illustration only — your lender's figures are the ones that count.
Frequently asked questions
How much can I overpay without a penalty?
Most fixed-rate UK mortgages allow overpayments of up to 10% of the outstanding balance each year without an early repayment charge, though the exact allowance and how it is measured varies. Trackers and standard variable rate deals often have no limit. Check your mortgage offer before setting up a regular overpayment.
Should I shorten the term or reduce the monthly payment?
Keeping the payment the same and shortening the term saves the most interest, because every extra pound goes straight to the balance. Reducing the monthly payment gives you breathing room now but leaves more interest to pay overall. This calculator models the term-shortening approach.
Is overpaying better than saving?
Compare your mortgage rate with the after-tax interest you could earn on savings. If your mortgage rate is higher, overpaying usually wins; if a savings account pays more after tax, saving may be better. Keep an emergency fund either way, because money in a mortgage is hard to get back.
Does a lump sum or a monthly overpayment help more?
A lump sum paid early removes interest for the whole remaining term, so pound for pound it usually saves more than the same amount spread over years. Regular overpayments are easier to budget and still compound their effect over time.
Will my lender recalculate my payment automatically?
Not always. Some lenders apply overpayments immediately and keep the payment level, others reduce your monthly payment at the next annual review, and a few only credit overpayments once a year. Ask your lender to apply overpayments daily and to keep the term shortening.
What happens when my fixed rate ends?
Your rate, and therefore your payment and interest saving, will change. This calculator assumes one interest rate for the whole term, so re-run it after each remortgage with your new balance, rate and remaining term.
Related calculators
Official sources
Figures on this page follow official UK government guidance. Rates last verified: 27 August 2026. See our data sources and calculation methodology.
Estimates for general guidance only — not personalised financial advice.