Mortgage Overpayment Calculator 2026

Overpaying your mortgage reduces the outstanding balance immediately, which means less interest accrues on the remaining amount for the rest of the term — this compounds over time, so overpayments made earlier in the mortgage save more interest than the same amount paid later.

Most lenders allow you to overpay up to 10% of the outstanding balance per year without an early repayment charge (ERC); overpaying beyond that limit while still in a fixed-rate deal typically triggers a penalty, often 1-5% of the amount over the limit. Always check your specific mortgage's ERC terms before making a large overpayment.

Worked example: on a £200,000 mortgage at 4.5% over 25 years, overpaying by £200/month from the start could save roughly £25,000–£35,000 in total interest and shorten the term by 4-5 years, depending on exactly when the overpayments begin and how the lender applies them (reducing the term vs reducing the monthly payment).

Whether overpaying is the right move also depends on your other financial priorities — some people are better off building an emergency fund or maximising pension contributions (which get tax relief) before overpaying a mortgage, especially at lower interest rates. This calculator estimates the interest saved; it doesn't account for early repayment charges, which you should check separately with your lender.