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UK · Mortgage tool

Mortgage overpayment calculator

See how much sooner your mortgage clears — and how much interest you keep — by overpaying. We also show what the same money could do as a deposit toward your next deal.

Your mortgage

What you still owe today — not the original loan.

5.00%
Rates as of Apr 2026
25 years

Extra you'd pay on top of your normal monthly payment.

A single overpayment made now — e.g. a bonus or savings.

Contractual payment: £1,169/mo (principal + interest). Overpayments are on top of this.

Mortgage clears early by
6 years 2 months
Down from 25 years to 18 years 10 months.
Total interest saved
£41,843
Interest falls from £150,754 to £108,911.
New payoff term
18 years 10 months
Interest (overpaid)
£108,911
Contractual P&I
£1,169

What this means for you

  • WINOverpaying £200/mo clears your mortgage 6 years 2 months early, saving £41,843 in interest.
  • TIPYou're within the 10% penalty-free overpayment allowance — about £17,600 of headroom left this year (£20,000 cap on this balance).
  • TIPSame money, leveraged instead: £200/mo set aside for 10 years is £24,000 — a 25% deposit toward roughly £96,000 of your next BTL. Overpaying is a guaranteed 5.00% saving; leveraging aims higher but carries more risk.
Interest saved
Saved = Interest(no overpay) − Interest(with overpay)
Each month we charge interest on the balance, subtract your payment plus any
overpayment
, and repeat until the balance reaches zero — standard reducing-balance
amortisation
. The saving is the difference in total interest between paying normally and overpaying. Watch the
ERC
allowance (usually 10% of the balance a year).
Estimates only, not financial advice. Assumes your interest rate stays constant for the remaining term and overpayments reduce the term (not the monthly payment). Check your lender's
early-repayment
terms before overpaying near the 10% allowance. Looking at the wider picture? See the full mortgage calculator.

Common questions

How does overpaying my mortgage save money?

Every overpayment comes straight off your outstanding balance, so interest is charged on a smaller amount from then on. That means the mortgage clears sooner and you pay less interest in total. Overpaying early in the term saves the most, because that's when the balance — and so the interest — is highest.

What is the 10% overpayment allowance (ERC)?

Most fixed and discount mortgage deals let you overpay up to 10% of the outstanding balance each year with no penalty. Overpay more than that and lenders can apply an early-repayment charge (ERC), often 1–5% of the excess. On a standard variable rate there's usually no cap. Always check your specific mortgage terms.

Should I overpay or invest the money instead?

Overpaying is a guaranteed, tax-free return equal to your mortgage rate. Investing — including using the cash as a deposit on another buy-to-let — can return more but carries risk and isn't guaranteed. This calculator shows both: the interest you'd save by overpaying, and what the same monthly amount could put toward your next property deposit.

Is a lump sum or a monthly overpayment better?

Both reduce the balance; a lump sum paid early saves the most interest because it cuts the balance sooner, while regular monthly overpayments are easier to sustain. You can model either or both here — just keep an eye on the annual 10% allowance.