This mortgage overpayment calculator works out how much interest you save by making an extra payment on your mortgage, whether you use it to lower the monthly payment or to shorten the remaining term.
Lower the payment or shorten the term
When you overpay capital on a mortgage, you can either lower the payment (same term, you pay less each month) or shorten the term (same payment, you finish earlier). Shortening the term almost always saves more interest; lowering the payment improves your monthly cash flow.
The assumptions
This tool uses a French (equal-instalment) amortization system with a constant rate. It doesn’t include early-repayment fees or taxes — for guidance only, not financial advice.