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Overpaying a repayment mortgage reduces the balance that interest is charged on, so every extra pound works twice: once against the balance and again against the interest that balance would have generated for the rest of the term.
Enter your balance, rate and remaining term, then add a regular monthly overpayment, a lump sum, or both. The calculator simulates the mortgage month by month and shows how much sooner it clears and how much interest that avoids.
Last reviewed
Estimated interest saved
£22,936.25
Many fixed-rate deals cap penalty-free overpayments, often at 10% of the balance each year. Check your offer for early repayment charges before committing.
The contractual monthly repayment is what the lender would charge on the balance, rate and remaining term you entered. Overpayments sit on top of that figure, not instead of it.
Interest saved is the difference between the total interest charged if you keep paying only the contractual amount and the total charged once your overpayments are applied. Time saved is how many months earlier the balance reaches zero.
A repayment mortgage is a standard amortising loan. Each month interest is added at one twelfth of the annual rate, then the payment is deducted. The remaining balance carries forward.
Any lump sum is applied immediately, before the first month of interest. The regular overpayment is added to the contractual payment every month until the balance reaches zero, at which point the final payment is reduced to whatever is left.
£200,000 outstanding at 4.5% with 20 years remaining, overpaying £200 a month.
The mortgage clears several years early and the interest saved runs into five figures.
Current mortgage balance (£): 200000; Annual interest rate (%): 4.5; Remaining term (whole years): 20; Additional months on the term: 0; Regular monthly overpayment (£): 200; Lump sum paid now (£): 0
Estimated interest saved: £22,936.25
The result depends on the following. If any of them do not match your situation, treat the figure as indicative only.
The calculator runs two simulations on the same opening balance: one with the contractual payment alone and one with the lump sum and monthly overpayment applied. The difference between the two gives the interest and time saved.
A zero interest rate is handled as a straight division of the balance across the remaining months, so the result stays a real figure rather than a division by zero. All arithmetic runs locally in your browser.
Overpaying while keeping the same monthly payment shortens the term and saves the most interest. Asking the lender to recalculate the payment instead lowers your monthly commitment but keeps the original end date. This calculator models the first approach.
A lump sum paid today avoids interest on that amount for the whole remaining term, so pound for pound it saves more than the same money spread over several years. Regular overpayments are usually easier to sustain.
Most UK lenders allow some penalty-free overpayment, commonly up to ten per cent of the balance each year during a fixed deal, with no restriction on a standard variable rate. Check your mortgage offer, because early repayment charges are set per product.
Please note: Results are estimates for general information only and are not financial, tax or legal advice. Figures depend on the assumptions listed on each page and on your personal circumstances.
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