Remortgage Lab

Mortgage overpayment calculator

Last checked UK · illustrative onlyNot a lender or brokerAboutDisclaimer

Published by Rodway Labs — not a mortgage adviser

See how monthly or one-off overpayments reduce interest and term. Enter your annual overpayment allowance — exceeding it can trigger an ERC.

Equations and method

Assumptions

  • EXAMPLE balance/rate/term starters (£185k / 4.19% / 22y) for consistency with other tools.
  • Overpayments reduce term (not payment) by default.
  • Annual allowance is user-entered — exceeding it can trigger an ERC. Check your offer.
  • Does not model stepped ERCs or cashback clawback.

Your inputs

Starter figures are UK EXAMPLE values you can edit. Scenario rates, not a market quote. 4.19%, 4.39%, 4.14% and 7.49% SVR on this site are labelled EXAMPLE figures, last checked 2 November 2026. Results update as you type.

Results

Baseline term

22 years

New term

16 years 11 months

Months saved

61

Interest saved

£24,942

Year-1 overpayments

£2,400

Allowance amount

£18,500

Year-1 overpayments look within your entered allowance on these inputs. Exceeding the allowance later can still trigger an ERC — check your offer.

Method: month-by-month amortisation with extra capital each month (and optional one-off in month 1), compared with a baseline schedule. Exceeding the annual overpayment allowance can trigger an ERC — check your offer.

Illustrative only — not personalised advice, not a credit or mortgage offer, and not an FCA-regulated recommendation. Your offer and ESIS set the real ERC, fees and terms. Full calculator disclaimer.

What to do next

How to use this calculator

  1. Enter your balance, rate and remaining term.
  2. Enter a regular monthly overpayment, a one-off overpayment, or both.
  3. Enter your annual overpayment allowance (often 10% of the balance) so the tool can flag overpayments that could trigger an ERC.
  4. Read the new term, months saved and interest saved, compared with no overpayments.
  5. Check that year-1 overpayments stay within the allowance amount shown before you pay anything extra.
Methodology and equations

Month-by-month amortisation with extra capital reduction each month (and optional one-off in month 1), compared with a baseline schedule at the same rate and term.

The assumptions are listed in the box near the top of this page.

How this calculator works

Use this to see how a regular or one-off overpayment changes interest and term on the rate you enter. It does not say whether overpaying beats switching.

How this is calculated

  1. The normal payment comes from the standard repayment formula on your balance, rate and remaining term.
  2. The tool then runs the schedule month by month: interest = balance × rate ÷ 12, and the payment plus any regular overpayment goes to interest first and capital second. A one-off amount is paid in month 1.
  3. The payment does not change, so overpaying shortens the term. Interest saved and months saved are the differences from the schedule without overpayments.
  4. Allowance = balance × the percentage you enter. A note appears when the one-off plus 12 months of regular overpayments exceeds it.

This matches the code behind the tool. The full formulae are on the calculator methodology page.

Worked example

Scenario rates, not a market quote

EXAMPLE house: £185,000, 22 years, 4.19%. Extra payments reduce later interest if the lender applies them to capital.

If the allowance is 10% a year, 10% of £185,000 is £18,500. Above that, an ERC can apply. Scenario only, last checked 2 November 2026.

With the starter £200 a month on the same EXAMPLE, interest falls from £98,485.34 to £73,543.47, saving £24,941.87, and the mortgage ends 61 months sooner. That is £2,400 in year one, well inside an £18,500 allowance. A one-off £25,000 would exceed it and is flagged.

Limitations: what this ignores

  • Whether the allowance is of the original or current balance.
  • Offset mortgages.
  • Returns on keeping the cash.
  • A later product transfer.

Sources

Guides that go with this tool

Questions

Will overpaying always cut the term?
Only if the lender applies the extra to capital and you do not stretch the term.
Can an overpayment trigger an ERC?
Yes, above the allowance in the offer.
Should I overpay or remortgage?
Different questions. This site does not recommend either.
Does this include fees?
No. Fees and ERCs for leaving belong in the true-cost calculator.
Are starter rates live?
No. 4.19% is the EXAMPLE current rate.

Notes & FAQs

Do overpayments reduce the term or the payment?
This tool assumes overpayments reduce the term while the contractual payment stays the same — a common default. Some lenders let you reduce the payment instead.
What if I exceed the allowance?
Many deals allow ~10% of the balance per year without an ERC. Overpaying beyond the allowance can trigger an early repayment charge on the excess — check offer wording.

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