Mortgage overpayment calculator
Overpaying your mortgage clears the balance faster, so you pay interest on a smaller amount for less time. On a £200,000 balance at 4.5% over 25 years, an extra £150 a month saves roughly £29,000 in interest and clears the mortgage almost 5 years early; a £15,000 lump sum on a £150,000 balance at 4.75% over 20 years saves roughly £21,000 and over 3 years. This free calculator models either a regular monthly overpayment or a one-off lump sum. It runs in your browser, needs no email, and stores nothing. Check your overpayment allowance first to avoid an early repayment charge.
See what overpaying saves you
Free. No email needed. Runs in your browser, stores nothing.
Interest saved
£29,284
Time cut off the term
4 years and 11 months
Worked example: on a £200,000 balance at 4.5% over 25 years, the normal payment is about £1,112 a month. Adding £150 to make it £1,262 clears the mortgage in about 20 years and 1 month instead of 25 years, saving roughly £29,284 in interest. Change the figures above for your own mortgage.
Simplified illustration for a repayment mortgage at a rate assumed fixed for the whole term. It ignores fees and future rate changes, and assumes your lender allows the overpayment with no early repayment charge. Not a quote and not advice. A regulated broker or your lender can confirm your real figures.
The formula
Your normal monthly payment on a repayment mortgage comes from the standard loan-amortisation formula:
Monthly payment = P × r × (1 + r)n ÷ ((1 + r)n − 1)
where P is the outstanding balance, r is the interest rate divided by 12, and n is the number of months left.
A monthly overpayment is added to that payment every month. The loan is then paid off in fewer months, found by solving the same formula backwards for the new, shorter n at the higher payment. A lump sum works differently: it comes straight off the balance once, so P falls immediately and the same monthly payment then clears the smaller balance in fewer months. Either way, the interest saved is the difference between the total interest you would have paid at the original term and the total interest actually paid at the new, shorter one.
Three worked examples
These are the calculator's own maths, worked through by hand so you can follow it with the tool switched off.
A. Regular monthly overpayment
A £200,000 balance at 4.5% with 25 years left has a normal payment of about £1,112 a month. Adding £150 a month brings it to £1,262. That extra amount clears the balance in about 20 years and 1 month instead of 25 years, a saving of 4 years and 11 months. Total interest falls from about £133,499 to about £104,216, a saving of roughly £29,284.
B. One-off lump sum
A £150,000 balance at 4.75% with 20 years left has a normal payment of about £969 a month. Putting a £15,000 lump sum straight onto the balance, while keeping the same monthly payment, clears the mortgage in about 16 years and 11 months instead of 20 years, a saving of 3 years and 1 month. Total interest falls from about £82,641 to about £61,628, a saving of roughly £21,013.
C. A larger balance and overpayment
A £350,000 balance at 4.25% with 30 years left has a normal payment of about £1,722 a month. Adding £400 a month brings it to £2,122. That clears the mortgage in about 20 years and 8 months instead of 30 years, a saving of 9 years and 4 months. Total interest falls from about £269,844 to about £176,689, a saving of roughly £93,156. On a longer, larger mortgage the same style of overpayment does noticeably more, because more of the early payments are interest.
All three assume a repayment (not interest-only) mortgage, a rate fixed for the whole period, no fees, and no early repayment charge. They are illustrations of the method, not a quote for any specific mortgage.
Why a small overpayment does so much
On a repayment mortgage, most of your early payments are interest, not capital. An overpayment, whether monthly or a lump sum, goes straight onto the capital, so it removes future interest that would otherwise have built on that money for years. The longer the remaining term and the higher the rate, the more each pound of overpayment is worth. That is why overpaying early in a mortgage saves far more than the same overpayment near the end. See the repayment calculator to see how the balance falls over time.
Check these before you overpay
- Your overpayment allowance. Many UK mortgage deals let you overpay up to a set amount each year without penalty, commonly around 10% of the balance, though this varies by lender and deal and is set out in your mortgage offer, not by any general rule.
- Early repayment charges. Overpay beyond your allowance during a fixed or discounted period and you may trigger an early repayment charge (ERC). ERCs are commonly in the region of 1% to 5% of the amount over the allowance, and often taper down over the deal, but the real figure and the years it applies are specific to your mortgage. Always check your offer or ask your lender before sending a large lump sum. See our early repayment charges guide.
- Your safety net. Keep accessible emergency savings first, because money overpaid is hard to get back.
- Costlier debts. Clearing high-interest credit cards or loans usually beats overpaying the mortgage.
- Overpay or save. Weigh your mortgage rate against your savings rate: see should I overpay or save.
Common questions
How does overpaying a mortgage save money?
Overpaying reduces the outstanding balance faster, so you are charged interest on a smaller amount for the rest of the term. Because interest builds on the balance every month, taking money off early stops years of interest before it ever accrues. The effect is largest when your rate is high and your term is long.
Is it better to overpay each month or pay a lump sum?
Both help, and this calculator models either one. A regular monthly overpayment is the simplest habit and compounds steadily. A single lump sum saves the most when it is paid early, because the money comes off the balance sooner and stops that portion of interest accruing straight away. Choose whichever matches how the money actually arrives: a bonus or inheritance suits the lump sum mode, spare income each month suits the regular mode.
Will I be charged for overpaying?
Many deals let you overpay up to a set amount each year without penalty, often around 10% of the balance. Overpay beyond that during a fixed or discounted period and you may trigger an early repayment charge, commonly somewhere in the region of 1% to 5% of the amount over the allowance, tapering over the deal. The exact figure is in your mortgage offer, not a general rule, so check it or ask your lender before you send a large lump sum. See our early repayment charges guide.
Should I overpay or keep the money in savings?
As a rough guide, overpaying wins when your mortgage rate is higher than the after-tax interest your savings would earn. But keep an emergency fund first, since money overpaid is hard to get back, and clear costlier debts before either. Our overpay or save answer walks through the trade-off.
Coming up to a deal ending? See remortgage, or work out a payment change with the payment-shock calculator.
Founder, MortgageExplained
Adam spent nearly a decade as a mortgage adviser at Just Mortgages, with further experience in commercial finance. He is CeMAP and CF qualified. He built MortgageExplained to do one thing well: explain mortgages in plain English, then introduce you to a regulated broker when you are ready. Every page is written and reviewed by Adam.
Last reviewed: 3 August 2026