Mortgage Overpayment Calculator
See how much interest you save — and how many years you shave off your term — by overpaying your UK mortgage.
Check your lender's ERC-free overpayment allowance (usually 10%/yr).
On a £0 mortgage, overpaying £0/month saves £0 in interest and clears the mortgage 0m earlier.
How overpayment size affects the outcome
How mortgage overpayments work
Interest compounds on the balance
Every month your lender charges interest on your outstanding balance. Overpayments reduce that balance, so next month's interest is lower. The saving compounds over the remaining term.
Check the 10% rule
Most fixed-rate mortgages let you overpay up to 10% of the balance each year with no Early Repayment Charge (ERC). Go over, and you'll typically pay 1–5% ERC on the excess.
Frequently asked questions
How much can I overpay on my mortgage without penalty?
Most UK lenders let you overpay up to 10% of the outstanding balance each year without an Early Repayment Charge. Some lenders (e.g. tracker or offset products) allow unlimited overpayments. Always check your mortgage offer.
Is it better to overpay or save?
Generally yes if your mortgage rate exceeds the after-tax return on your savings. On a 4.6% mortgage, every £1 overpaid saves ~4.6% — hard to beat with a risk-free savings account. But keep an emergency fund first.
Should I shorten the term or reduce the monthly payment?
Overpaying shortens the term by default — you pay the same and clear sooner. Some lenders let you use overpayments to reduce future monthly payments instead. Shortening the term saves more interest overall.
Does a lump sum save more than monthly overpayments?
A lump sum saves more per pound because it reduces the balance immediately. But regular overpayments are easier to sustain. Combine both where possible.
Will my lender change my direct debit after an overpayment?
Depends on the lender and product. Fixed-rate lenders usually keep the DD the same and reduce the term. Tracker/variable lenders often recalculate and lower the DD. Ask your lender which method they use.
Reviewed by the Mortgage118 editorial team. Calculation uses a monthly amortisation model with end-of-month payments. Individual lender methods vary; check your mortgage offer.