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Mortgage overpayment calculator

See how regular or one-off extra payments could change the interest paid and the time left on a repayment mortgage. The result assumes the rate and contractual payment stay unchanged.

Your current mortgage

Compare the entered payment with regular and one-off overpayments.

The mortgage balance today.

Assumed unchanged.

Whole years remaining.

Your contractual payment before extras.

Overpayments

Paid from the first month.

Applied before that month’s interest.

Month 1 is the next payment.

Adds a completion date.

Illustrative overpayment result

£42,330.46

estimated interest saved

Time saved
6 years 9 months
Revised term
18 years 3 months
Interest with extras
£91,167.75
Completion
Add a start month for a calendar date

Important: Check your lender’s overpayment allowance and any early repayment charge before paying extra.

Balance comparison
PointWithout extrasWith extras
Year 1£195,569.32£188,100.44
Year 2£190,935.09£180,672.98
Year 3£186,087.97£172,904.29
Year 4£181,018.17£164,778.71
Year 5£175,715.47£156,279.85
Year 10£145,316.19£107,557.65
Year 15£107,262.51£46,567.41
Year 20£59,627.06£0.00
Year 25£0.00£0.00

Calculation mortgage-overpayment@1.0.0. Overpaying is not always the best use of available money.

Assumptions used

  • The interest rate and contractual monthly payment stay unchanged.
  • Interest is charged monthly at the annual rate divided by 12.
  • The one-off overpayment is applied at the start of the selected month.
  • Monthly overpayments start immediately and stop when the balance reaches zero.
  • Calculations exclude fees, early repayment charges and lender overpayment limits.

How the comparison works

The engine runs two month-by-month balance simulations with unrounded values. The baseline uses only the contractual payment. The comparison applies the monthly extra immediately and the one-off extra at the start of its selected month.

Each month’s interest is the opening balance multiplied by the annual rate divided by 12. A final payment is reduced when it would otherwise exceed the balance plus interest.

Worked example

For the example £200,000 balance at 4.5% with 25 years remaining, adding £200 a month and £5,000 in month 12 produces estimated interest savings of £42,330.46 and a time saving of 6 years 9 months. The revised illustrative term is 18 years 3 months.

This text is generated from calculation version mortgage-overpayment@1.0.0, the same engine used by the form.

What this result does not tell you

It does not decide whether overpaying is right for you. Keeping accessible savings, repaying higher-cost debt or contributing elsewhere may matter more in your circumstances.

  • Your lender may limit penalty-free overpayments.
  • Early repayment charges and product changes are excluded.
  • Daily-interest timing and lender rounding may produce a different figure.
  • A low contractual payment that does not clear the balance prevents a reliable savings comparison.

Your home may be repossessed if you do not keep up repayments on your mortgage.

Sources and change log

The calculation contains no lender rate feed: every rate and payment comes from the visitor. The information boundary follows FCA MCOB 5.4.

20 August 2026: initial month-by-month overpayment engine and browser calculator added.

Questions people ask

When is the one-off payment applied?

At the start of the selected payment month, before that month’s interest. Your lender’s exact timing may differ.

Why can interest saved be unavailable?

If the entered contractual payment leaves a balance at the end of the selected term, there is no complete baseline payoff to compare with.