Take a £200,000 balance at 5 percent with 20 years left, on which the normal repayment is about £1,320 a month. Left alone, that loan costs roughly £116,779 in interest over the full term. Add a £200 monthly overpayment and two things happen: the interest bill falls to about £90,489, and the mortgage clears around 48 months, four years, early. The overpayment saves close to £26,290 in interest because every extra pound goes straight at the capital, so future interest is charged on a smaller balance. The effect is largest in the early years when the balance, and therefore the interest, is at its highest.
Scenario
Interest paid
No overpayment (20 years)
£116,779
£200 per month overpayment
£90,489
Time shaved off term
48 months
Interest saved
£26,290
How it is calculated
The tool first works out your normal monthly repayment from the balance, rate, and remaining term using the standard amortising formula. It then simulates the mortgage month by month twice: once at the normal payment and once with your overpayment added on top. Each month it charges interest on the outstanding balance at one twelfth of the annual rate, then subtracts the payment, so any surplus reduces the capital directly. The difference in total interest between the two runs is the saving, and the difference in the number of months is the time cut from the term. Most fixed-rate UK mortgages allow penalty-free overpayments of up to 10 percent of the balance each year, so check your lender's cap before committing, since exceeding it can trigger an early repayment charge. The model assumes the rate stays constant and ignores any cap, so treat it as an illustration of the mechanics rather than a precise forecast.
Frequently asked questions
Overpayment limits?
Most fixed-rate UK mortgages allow up to 10 percent of the outstanding balance per year as penalty-free overpayment. Above that triggers Early Repayment Charges (typically 1-5 percent of the overpaid amount).
Does overpaying reduce my monthly payment or shorten the term?
With most UK lenders, overpayments reduce the outstanding capital but your required monthly payment stays the same. The mortgage simply clears earlier because less interest accrues each month. Some lenders do offer a recalculation option that lowers your required payment instead, so check your mortgage terms.
Is overpaying always the best use of spare cash?
It depends on your interest rate versus the returns you could earn elsewhere. If your mortgage rate is higher than the after-tax return on savings or investments, overpaying usually wins. However, many fixed-rate mortgages in 2025 and 2026 sit above 4 percent, making overpayment a reliable, risk-free return that is hard to beat in a cash savings account.
How do I notify my lender of a one-off overpayment?
Most UK lenders accept lump-sum overpayments via online banking or telephone, referencing your mortgage account number. Always request written confirmation that the payment has been applied to capital rather than future interest. Some lenders require you to state it is a capital overpayment at the time of transfer to ensure it is processed correctly.