Free calculator · 2026/27 rates

Mortgage overpayment calculator

Overpaying a mortgage cuts the interest you pay and the years you pay it for, because interest is charged on a smaller balance every month afterwards. Overpaying £100 a month on a £200,000 mortgage at 4.5% over 25 years saves £21,142 in interest and clears it 3 years 6 months early. Enter your own figures below.

Your mortgage

Outstanding balance

Interest rate

Remaining term

Monthly overpayment

One-off lump sum

Assumes a repayment mortgage with the rate fixed for the whole term and overpayments applied to reduce the term.

Overpayment result

Interest saved

£21,142

Mortgage-free sooner by

3 years 6 months

Standard monthly payment: £1,111.66
New monthly outgoing: £1,211.66
Total interest, no overpaying: £133,499
Total interest, overpaying: £112,358
Paid off in: 25 years
Paid off with overpayments: 21 years 6 months

Balance at the end of each year
Year	Balance (no overpaying)	Balance (overpaying)	Interest saved that year
1	£195,569	£194,344	£25
2	£190,935	£188,429	£81
3	£186,088	£182,242	£140
4	£181,018	£175,770	£202
5	£175,716	£169,001	£266
10	£145,317	£130,197	£635
15	£107,264	£81,622	£1,097
20	£59,629	£20,817	£1,676
25	£0	£0	£320

How do mortgage overpayments save interest?

A repayment mortgage charges interest each month on the balance still owed. Your contractual payment covers that month’s interest plus a little capital. Anything you pay on top goes straight to capital, so next month’s interest is charged on a smaller balance. The effect compounds: each overpayment saves interest every month for the rest of the term, which is why early overpayments are worth more than later ones.

Overpayment on £200,000 at 4.5% over 25 years	Interest saved	Term cut
£50 a month	£11,534	1 years 10 months
£100 a month	£21,142	3 years 6 months
£200 a month	£36,280	6 years 1 months
£300 a month	£47,708	8 years 1 months
£10,000 lump sum now	£19,300	2 years 2 months

Worked example: £100 a month extra

The standard payment on £200,000 at 4.5% over 25 years is £1,111.66 and the total interest over the term is £133,499. Adding £100 a month takes the outgoing to £1,211.66, clears the loan in 21 years 6 months instead of 25 years and cuts total interest to £112,358. You pay £312,358 in total rather than £333,499.

What should you check before overpaying?

Early repayment charges. Most fixed deals allow 10% of the balance per year penalty-free. Above that, charges of 1% to 5% can wipe out the saving.
How the lender applies it. Ask for the term to be reduced, not the payment. Reducing the payment saves far less interest.
Emergency savings first. Overpaid money is hard to get back. Keep three to six months of expenses accessible before overpaying.
Higher-rate debts. Clear credit cards and loans charging more than the mortgage rate before overpaying the mortgage.

Buying? Work out the tax on the purchase first with the stamp duty calculator.

Frequently asked questions

How much can I overpay on my mortgage without a penalty?

Most fixed and tracker deals allow overpayments of up to 10% of the outstanding balance each year without an early repayment charge. Some lenders allow more, and once you are on the standard variable rate there is usually no limit. Check your mortgage offer or ask your lender before making a large lump sum payment.

Is it better to overpay monthly or with a lump sum?

Pound for pound, money paid earlier saves more interest, so a lump sum now beats the same total spread over the year. In practice the difference is small; what matters most is the total overpaid and doing it consistently. If you are choosing, keep an emergency fund first and then overpay.

Should I overpay my mortgage or save?

Compare your mortgage rate with the after-tax interest you can earn on savings. If the mortgage rate is higher, overpaying gives a guaranteed return equal to that rate. If savings pay more, or you would exhaust your emergency fund, saving is usually better. Overpaying is also less flexible because the money is hard to get back.

Does overpaying reduce my monthly payment or the term?

That depends on what you ask the lender to do. Most lenders default to keeping your payment the same and shortening the term, which saves the most interest. Some let you reduce the monthly payment instead, which lowers your outgoings but saves less overall. This calculator assumes the term is reduced.