Mortgage Overpayment Calculator UK 2026

A mortgage overpayment is the extra money you pay on top of your contractual monthly payment. Because UK repayment mortgages charge interest on the outstanding balance, every extra pound goes straight at the capital — and that pound then stops accruing interest for the whole remaining term. That is why a modest £150 or £250 a month can knock years off the end date rather than just a few months.

This calculator runs a full month-by-month amortisation twice. The first run uses your contractual payment only, based on your balance, rate and remaining term. The second run adds your overpayment to every month and re-amortises until the balance hits zero. The difference between the two total-interest figures is your saving, and the difference in months is how much earlier you are mortgage-free.

Two real-world caveats. Most UK lenders cap penalty-free overpayments at 10% of the outstanding balance per year while you are inside a fixed deal — go over and you may pay an early repayment charge, so check your offer document. And the model assumes your interest rate stays put for the remaining term, which almost never happens in the UK's two-to-five-year fix market. Re-run it each time you remortgage.

Overpaying is not automatically the right call. If you hold credit card or car finance debt at a higher rate, clear that first. If your savings account pays more after tax than your mortgage rate costs, saving wins. And never overpay away your emergency fund — money in the mortgage is very hard to get back out.