Mortgage Overpayment Calculator
Making overpayments on your mortgage — even small ones — can save you a significant amount of interest and cut years off your term. Use the tabs below to model monthly overpayments or a one-off lump sum.
How much could you save?
Enter your current mortgage details and your planned overpayment below.
Related: Mortgage Payment · Mortgage Affordability
How the mortgage overpayment calculator works
When you make an overpayment on a repayment mortgage, the extra money reduces your outstanding balance immediately. Because your standard monthly payment stays the same, a greater proportion of each future payment goes toward paying off the principal rather than interest — shortening your term and cutting the total interest you pay. Even modest monthly overpayments of £100–£200 can shave years off a 25-year mortgage.
Most UK lenders allow overpayments of up to 10% of the outstanding balance per year without an early repayment charge (ERC). This calculator models both monthly overpayments and one-off lump sums, showing you the interest saved and the new payoff date. Always check your mortgage terms before overpaying — if you are in a fixed or discounted rate period, exceeding the annual allowance can trigger a penalty fee.
Before overpaying, it is worth ensuring you have a sufficient emergency fund and have cleared any higher-interest debts such as credit cards or personal loans. For those with flexible mortgages or offset mortgages, your lender may offer additional options not covered by this calculator.
Frequently asked questions
How much can I overpay my mortgage?
Most UK mortgage lenders allow overpayments of up to 10% of the outstanding balance per year without incurring an early repayment charge (ERC). This limit typically resets each calendar or mortgage year. Always check your mortgage terms, as some lenders have different thresholds or charge a fee if you exceed them.
Does overpaying reduce the term or the monthly payment?
By default, most UK lenders apply overpayments to reduce your mortgage term while keeping the monthly payment the same. Some lenders may instead reduce your monthly payment. Reducing the term is generally the better option as it saves more interest overall, but you can often contact your lender to choose which approach suits you.
Is there a penalty for overpaying a mortgage?
Yes, if you exceed your lender's annual overpayment allowance (usually 10%) during a fixed or discounted rate period, an early repayment charge (ERC) will typically apply. ERCs are usually a percentage of the amount overpaid above the threshold, often between 1% and 5%. There are generally no penalties on standard variable rate (SVR) mortgages.
When is the best time to start overpaying?
The earlier in your mortgage term you start overpaying, the greater the savings, because interest is charged on a higher balance in the early years. However, it is also worth ensuring you have an adequate emergency fund before directing surplus income to mortgage overpayments. If you have higher-interest debts such as credit cards or personal loans, it usually makes financial sense to clear those first.