Mortgage Overpayment Calculator UK

See exactly how much interest and time you'll save by overpaying your UK mortgage. Compare monthly overpayments, one-off lump sums and combined strategies.

Your Mortgage & Overpayments
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Extra amount paid each month on top of your normal payment.
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A single lump sum overpayment, applied immediately.
How Overpayments Save You Money

Every overpayment reduces your capital balance. Because mortgage interest is charged on the balance each month, a lower balance means less interest is added next month — and so on, compounding the saving for the rest of the mortgage term.

  • Monthly overpayments compound steadily and are easy to set up via standing order.
  • Lump sums have the biggest impact when paid early in the term.
  • Combine both to stay within your ERC-free allowance and maximise savings.
Frequently Asked Questions

Most UK lenders allow you to overpay up to 10% of your outstanding mortgage balance each year (during your fixed or tracker deal) without an Early Repayment Charge (ERC). Once your deal ends and you move onto the Standard Variable Rate (SVR), you can usually overpay any amount. Always check your mortgage offer for the specific allowance.

Compare your mortgage interest rate (after tax) with your savings or investment return (after tax). If your mortgage rate is higher than the best savings rate you can get, overpaying usually wins. Also consider keeping an emergency fund first, maximising any employer pension match and using your ISA allowance before overpaying.

Most lenders default to reducing the term, which saves the most interest overall. Reducing the monthly payment gives you extra cash flow but saves less interest. This calculator assumes you keep paying the original monthly amount and the term shortens automatically, which is the standard interest-saving strategy.

A lump sum paid early in the mortgage typically saves more interest because more of the balance is reduced for longer. However, regular monthly overpayments are often more achievable and, over the full term, can save very large amounts of interest. The best strategy is usually to do both within your annual ERC-free allowance.

No. Overpaying your mortgage does not harm your credit score. In fact, reducing your overall debt-to-income ratio can be viewed positively by lenders if you remortgage in future. Make sure you tell your lender that the overpayment is a "capital reduction" so it comes off your balance rather than being held as a credit.
Quick Tips
Stay within your ERC allowance

Usually 10% of your balance each year. Check your mortgage offer.

Tell your lender it's capital reduction

Some lenders treat overpayments as future payments unless you specify.

Keep your emergency fund

Aim for 3-6 months of expenses before throwing money at the mortgage.

Review when remortgaging

A lower balance can move you into a cheaper Loan-to-Value bracket.

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