Overpaying your mortgage: how much interest it actually saves

An extra £200 a month feels small next to a £1,389.58 monthly payment. Over the life of a 25-year mortgage it isn't – it can remove tens of thousands of pounds of interest and years off the term, because every pound of overpayment stops compounding immediately.

8 min read · Updated: 7 September 2026

Key points

On a £250,000 mortgage at 4.5% over 25 years (monthly payment £1,389.58), overpaying by £200 a month saves £38,458.01 in total interest and clears the loan 5 years 1 month early.
A smaller overpayment still counts: £100 a month on the same loan saves £21,870.88 in interest and shortens the term by 2 years 11 months.
A one-off lump sum works the same way: £10,000 paid after 5 years, with the payment kept the same afterwards, saves £13,819.55 in interest and finishes the loan 1.5 years early.
Overpaying early in the term saves more than the same amount overpaid later, because early balances carry more years of future interest.
Most UK lenders cap penalty-free overpayments at 10% of the outstanding balance a year during a fixed deal – check your mortgage offer before overpaying beyond that.

Why overpaying works: interest is charged on the balance

A repayment mortgage charges interest each month on whatever balance is still outstanding, then splits the fixed monthly payment between that interest and a slice of principal. Any extra amount paid on top comes off the principal immediately, which means every future month's interest is calculated on a smaller number. The saving isn't just the extra amount itself – it's that amount, plus all the interest it would otherwise have generated for every remaining month of the term.

This is why overpaying a mortgage is mathematically different from putting the same money in an ordinary savings account: the “return” on an overpayment is the mortgage's own interest rate, guaranteed, for as long as the loan would otherwise have run.

Worked example: £250,000 loan, 4.5%, 25 years

Take a £250,000 repayment mortgage at 4.5% over 25 years. The standard monthly payment is £1,389.58, and paid exactly to term it costs £166,874.69 in total interest – bringing the total repaid to £416,874.69. Adding a fixed amount to that payment every month changes both numbers substantially.

Effect of a regular monthly overpayment on a £250,000 loan at 4.5% over 25 years

Extra paid per month (£)	New monthly payment (£)	Time to clear the loan	Total interest paid (£)	Interest saved vs. no overpayment (£)
0 (baseline)	1,389.58	25 years 0 months	166,874.69	0.00
100	1,489.58	22 years 2 months	145,003.81	21,870.88
200	1,589.58	19 years 11 months	128,416.68	38,458.01

The pattern is not proportional: doubling the overpayment from £100 to £200 a month more than doubles the interest saved, from £21,870.88 to £38,458.01. That's because a bigger overpayment reduces the balance faster, which compounds the saving over a shorter remaining term.

Split of each year's payment between interest and principal, £250,000 loan at 4.5% over 25 years – interest dominates early, principal dominates later.

Why timing matters: early overpayments save more

The same £200 a month overpaid from month 1 saves more interest than if it started five years into the mortgage, because the balance in year 1 is close to the full £250,000 and has 25 years left to generate interest, while the balance in year 6 is smaller and has fewer years left. Every year an overpayment is delayed, it has less compounding time to work against.

A lump sum works the same way as a monthly overpayment

It doesn't have to be a recurring amount. A one-off £10,000 payment made at the 5-year mark on the same £250,000 loan – with the monthly payment left at £1,389.58 afterwards rather than reduced – saves £13,819.55 in total interest over the life of the loan and finishes it 1 year 6 months early.

Reduce the payment or reduce the term?

Most UK lenders offer a choice after a lump-sum overpayment: keep the monthly payment the same and finish the mortgage earlier (as shown above), or keep the original term and have the monthly payment recalculated downward instead. Reducing the term saves more interest overall, because the loan spends less total time accruing interest. Reducing the payment instead lowers monthly outgoings immediately, which matters more if cash flow, not total cost, is the priority.

£10,000 lump sum at year 5 on the £250,000/4.5%/25-year loan: two ways to apply it

Option	Monthly payment after the lump sum	Remaining term	Total interest saved
Keep the payment, shorten the term	£1,389.58 (unchanged)	18 years 7 months (was 20 years)	£13,819.55
Keep the term, lower the payment	Recalculated on a smaller balance over the same 20 remaining years	20 years (unchanged)	Smaller saving than shortening the term

Model your own overpayment

Enter your loan amount, rate and term, then add a monthly or one-off overpayment to see the exact interest saved and the new payoff date.

Go to the mortgage calculator

Checks to make before overpaying

Overpaying is only free if the mortgage terms allow it. Most fixed-rate deals in the UK market let you overpay up to 10% of the outstanding balance in any 12-month period without an early repayment charge (ERC) – overpaying beyond that allowance during a fixed deal typically triggers a percentage charge on the excess, which can easily outweigh the interest saved.

Check the current mortgage offer document (not a general assumption) for the exact penalty-free allowance – it's usually stated as a percentage of the balance at the start of the year, not the original loan amount.
Confirm whether the lender needs a written instruction to reduce the term versus the payment, or does it automatically – some default to shortening the term, others to lowering the payment.
Compare the mortgage rate to other uses of the same money, such as a higher-rate savings account or a workplace pension with employer matching – overpaying only wins financially if the mortgage rate is the higher guaranteed return.

Overpaying past the ERC allowance is rarely worth it

Early repayment charges are commonly 1% to 5% of the amount overpaid above the penalty-free allowance, tapering down each year of a fix. On a £20,000 excess overpayment at a 3% charge, that's £600 lost immediately – often more than a year of interest saved by paying it early instead of waiting until the fix ends.

When overpaying isn't the priority

Overpaying a mortgage is a guaranteed, risk-free return equal to the mortgage rate, but it's not automatically the best use of spare income. Higher-interest debt – credit cards or personal loans, which typically charge well above a mortgage rate – should normally be cleared first. An emergency fund covering a few months of essential costs is also worth having before committing to regular overpayments that can't easily be withdrawn again once paid in, since most lenders don't let you draw overpaid capital back out without a formal further-advance application.

Frequently asked questions

How much does overpaying £200 a month save on a mortgage?

On a £250,000 mortgage at 4.5% over 25 years, an extra £200 a month saves £38,458.01 in total interest and clears the loan 5 years 1 month early. The exact saving depends on your own balance, rate and remaining term.

Is it better to overpay early or later in the mortgage term?

Earlier. The same overpayment removes more future interest when the outstanding balance is larger and has more years left to run – a £200 monthly overpayment started in year 1 saves more than the same amount started in year 10.

Should I reduce my monthly payment or shorten the term after a lump sum?

Shortening the term while keeping the same monthly payment saves more total interest, because the loan accrues interest for less time overall. Reducing the payment instead lowers monthly outgoings sooner but saves less interest across the life of the loan.

How much can I overpay without a penalty?

Most UK lenders allow up to 10% of the outstanding balance per year penalty-free during a fixed-rate deal, but the exact figure is set in the mortgage offer, not a universal rule – check that document before overpaying a large amount.

What happens if I overpay more than the penalty-free allowance?

The excess is usually charged an early repayment charge (ERC), commonly 1% to 5% of the amount over the allowance and tapering down each year of the fix. This charge can outweigh the interest saved, so it's worth checking before overpaying past the limit.

Does a one-off lump sum work the same way as a monthly overpayment?

Yes. Both reduce the outstanding balance immediately, which lowers every subsequent month's interest charge. A £10,000 lump sum paid at year 5 of a £250,000/4.5%/25-year mortgage saves £13,819.55 in total interest if the payment stays the same afterwards.

Is overpaying always the best use of spare money?

Not automatically. Higher-interest debt such as credit cards should normally be cleared first, and most advisers suggest keeping an emergency fund before committing to overpayments, since overpaid capital usually can't be withdrawn again without a formal application to the lender.