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UK Mortgage Overpayment Calculator

See how much interest you can save and how much sooner you could clear your mortgage with a regular monthly overpayment.

Your mortgage

£
%
25 years
£

An extra amount each month on top of your normal repayment.

Interest saved over the term New term —
£—
Time cleared early£—
Original interest£—
Interest with overpayments£—
Paid off in — instead of —
Many UK lenders cap overpayments at 10% of the balance per year without an early-repayment charge. Check your mortgage terms.

This is a standard amortisation calculation with no UK-specific thresholds. Your actual terms (fees, overpayment limits, rate changes) depend on your lender. See Bank of England for the official Bank Rate.

How it works

Enter your outstanding balance, interest rate, remaining term and a monthly overpayment. The calculator simulates two repayment mortgages — one with and one without the extra payment — and shows how much interest you save and how much sooner you clear the loan. Because overpayments reduce the balance directly, they cut the interest charged on the remaining debt each month.

Example calculation

For an outstanding balance of £180,000 at 5% with 25 years remaining and a £100/month overpayment:

  • Original monthly payment: ≈ £1,052
  • Interest over 25 years without overpaying: ≈ £135,700
  • Interest with £100/month overpayment: ≈ £107,000
  • Interest saved: ≈ £28,700
  • Paid off in about 21 years 5 months — over 3 years earlier

Example is illustrative: real savings depend on your rate, term and whether you pay a lump sum instead. Overpayment limits and charges vary by lender.

Methodology & formula

The calculator simulates the loan month by month. Each month, interest is charged on the outstanding balance at the monthly rate (annual ÷ 12), then your normal repayment plus the overpayment reduces the balance:

Interest = balance × r   ·   new balance = balance + interest − (monthly payment + overpayment)
  • r = monthly interest rate = annual rate ÷ 12
  • Monthly payment = standard annuity payment for the original balance, rate and term
  • Overpayment = extra amount paid each month, applied directly to the balance

Interest saved = total interest without overpayments − total interest with overpayments. Time saved = original months − months to clear with overpayments.

Assumptions: constant rate for the whole term, no fees or charges, overpayments made monthly from day one. Lump-sum overpayments have a similar effect.

UK-specific information

  • Overpayment limits: most UK mortgage products allow overpayments of up to 10% of the balance each year without an early-repayment charge; above that you may pay a fee. MoneyHelper · 2026
  • Fixed deals: overpaying on a fixed-rate mortgage is usually allowed within the 10% limit; larger amounts may incur an ERC until the deal ends.
  • Reducing term vs reducing payment: keeping your payment the same and overpaying shortens the term and cuts interest, which is what this calculator models.
  • Weigh overpaying against other options — a higher-rate savings account or ISA can sometimes beat the interest you would save.

Frequently asked questions

Is overpaying my mortgage worth it?
Overpaying usually saves interest because it reduces the balance you are charged interest on. It is often worth it if your mortgage rate is higher than the after-tax return you could get on savings — but check your overpayment limit and any charges first.
Can I overpay any amount?
Most UK lenders allow up to 10% of the outstanding balance per year without an early-repayment charge. Above that, fees may apply, especially during a fixed-rate deal.
Should I overpay the mortgage or save instead?
Compare your mortgage rate with what you could earn after tax. If savings beat your mortgage rate, saving may be better; if the mortgage rate is higher, overpaying usually wins. Keep an emergency fund either way.
Does a lump sum save the same as monthly overpayments?
Both reduce the balance, but a lump sum saved earlier has a bigger effect because it cuts interest for longer. The calculator models regular monthly overpayments.
What happens if I overpay too much?
Above the lender's allowance you may pay an early-repayment charge, and some lenders may let you extend the term or reduce your payment instead of clearing the loan faster. Check your terms.
Does the calculator include fees?
No. Early-repayment charges, arrangement fees and other costs are not included — factor them into your decision.

Related calculators

UK Mortgage Calculator · UK Stamp Duty Calculator · UK Savings Calculator · Home

Sources

Casvia provides educational calculators and general information. It does not provide personal financial, tax or investment advice. Interest rates and product rules change — always check the latest official guidance.