Mortgage Overpayment Calculator

A mortgage overpayment is any amount you pay above your standard monthly repayment. Every extra pound you pay goes directly towards reducing your outstanding balance — which reduces the interest charged next month, and every month after that. Over a 20-year term, even a modest regular overpayment compounds into tens of thousands of pounds in savings.

The 10% rule

Most UK fixed-rate and tracker mortgages allow you to overpay up to 10% of your outstanding balance per year without incurring an early repayment charge (ERC). For a £200,000 mortgage, that is up to £20,000 per year — roughly £1,667 per month. If you overpay beyond this limit while your deal is active, your lender may charge a penalty, typically 1–5% of the excess amount. Always check the terms in your mortgage offer or Key Facts Illustration (KFI) before overpaying.

Once your fixed or tracker deal ends and you move onto your lender's Standard Variable Rate (SVR), the 10% cap usually no longer applies and you can typically overpay as much as you like.

When overpaying makes sense

Overpaying is most powerful early in your term, when your balance — and therefore the interest charged each month — is at its highest. If your mortgage rate is higher than the after-tax interest rate you can earn on savings, reducing your balance delivers a better guaranteed return than putting that money in the bank.

When to think twice

If your mortgage rate is low and you can earn more in a high-interest savings account, cash ISA, or investment account, it may make sense to save rather than overpay. Similarly, if you carry high-interest debt — credit cards or personal loans — clearing those first will usually save more money overall. And always maintain an accessible emergency fund before directing spare cash at your mortgage. Once money reduces your mortgage balance, you cannot easily access it again without remortgaging.