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Mortgage Overpayment Calculator

See how much interest you save and how many years you cut off your mortgage by overpaying each month. Free, shows the maths, no sign-up.

The loan
£
%
yrs
£
What overpaying does
Interest saved 0
New monthly payment—
Original payoff—
New payoff—
Time knocked off—
Balance: overpaying vs. not
Standard With overpayments

Assumes a level overpayment every month with no fee. Many fixed-rate deals cap penalty-free overpayments at ~10% of the balance per year — check your lender before committing.

How the maths works

The formula behind this calculator, with a worked example built from your own inputs.

Overpayment savingsOverpayment

There's no closed-form shortcut here — the tool runs the amortisation schedule twice and compares:

Interest saved = Interest(M) − Interest(M + extra)

Because the extra goes straight onto the principal, every £1 overpaid removes all the future interest that £1 would have generated. The loan ends the month the balance first hits zero, so the term shortens rather than the payment falling.

Worked example: £238,000 at 4.8% over 25y costs £171,120 in interest. Adding £200/month saves £41,482 and clears the loan in 19y 8m.
About the currency selectorCurrency

Changing the currency relabels the figures — it does not convert them. There are no exchange rates involved, and nothing is fetched from the internet.

If you enter 280,000 and switch from pounds to dollars, the tool reads it as 280,000 dollars, not as a converted amount. The maths is identical in every currency because percentages, ratios and time don't care about the unit — a 25-year loan at 4.8% has the same payment-to-principal relationship whether it's in yen or rand.

What does change is formatting, which follows each region's own conventions: where the symbol sits (€ goes after the number in German), which separators are used (Switzerland uses apostrophes, South Africa uses spaces), how digits are grouped (India groups in lakhs — 1,71,120 rather than 171,120), and whether decimals exist at all (yen and won have no minor unit).

So pick the currency you're actually working in and enter local amounts. Don't enter pounds and switch to dollars expecting a conversion.

Assumptions & what these formulas ignoreImportant

Interest compounds monthly on the outstanding balance — standard for UK repayment mortgages, though some lenders calculate daily, giving slightly different totals.

Rates are assumed fixed for the whole term. In reality a fixed period ends and you revert or remortgage, so long-term totals are illustrative.

Not included anywhere: arrangement and broker fees, stamp duty, early repayment charges, void periods, maintenance, letting agent fees, income or capital gains tax, and inflation. Figures are in today's money.

Overpayment caps: many fixed deals limit penalty-free overpayments to around 10% of the balance a year.

Investment returns assume future variability resembles the past — which markets regularly break. Depreciation curves are typical patterns, not valuations; real resale depends on mileage, condition, spec and demand.

These tools are for estimating and comparing options. They aren't financial advice, and for a decision that matters it's worth speaking to a qualified adviser or broker.

Frequently asked questions

Direct answers about this calculator.

How much does overpaying a mortgage save?

Overpayments come straight off the principal, so every unit overpaid removes all the future interest that amount would have generated. On a 238,000 balance at 4.8% over 25 years, adding 200 per month cuts total interest from about 171,120 to about 129,638 - a saving of roughly 41,482 - and clears the loan in 19 years 8 months instead of 25 years. Many fixed-rate deals cap penalty-free overpayments at around 10% of the balance per year.

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About these calculators

How the numbers are produced, what they are not, and where to get regulated advice.

How the numbers are produced

Every figure comes from a published formula shown openly in the How the maths works section, alongside a worked example built from your own inputs. Nothing is estimated, weighted or adjusted behind the scenes.

The calculations run entirely in your browser. Nothing is stored, no accounts, no tracking, no adverts, and no affiliate links to mortgage or insurance products. Nobody pays to appear here, so there is no incentive to nudge a result in any direction. The one time anything leaves your device is if you click Update to latest figures on the inflation tab — that makes a single request to the World Bank's open data API for newer price indices, and sends nothing about you. Every other figure, including the bundled inflation history, is already in the page.

Default rates and depreciation curves are typical illustrative figures, not live market data. The tools do not fetch interest rates, house prices, exchange rates or resale values from anywhere.

Limitations — read before relying on this

These are estimating tools for comparing options, not a substitute for professional advice. In particular:

· Figures exclude fees, stamp duty, early repayment charges, tax, void periods, maintenance and inflation.
· Rates are assumed fixed for the whole term; in reality fixed periods end and you revert or remortgage.
· Investment simulations assume future variability resembles the past — which markets regularly break. They show modelled uncertainty, not a forecast.
· Depreciation curves are typical patterns, not valuations. Real resale depends on mileage, condition, spec and demand.
· Changing currency relabels figures; it does not convert them.

For a decision that matters, speak to a qualified mortgage broker, accountant or FCA-regulated financial adviser. Nothing here is financial advice.

Sources & further reading

For impartial, regulated guidance in the UK:
· MoneyHelper — the government-backed free money guidance service
· MoneySavingExpert — mortgage guides and comparison tools
· Bank of England — current Bank Rate
· GOV.UK — Stamp Duty Land Tax
· FCA — checking an adviser is regulated

The amortisation, yield and depreciation formulas used here are standard published finance formulas, not proprietary methods — you are encouraged to check them against any other source.