How to calculate a mortgage overpayment: the definitive guide

Calculating an overpayment means estimating how your mortgage changes after an extra payment: how much the monthly payment falls or how many months you save, and above all how much interest you will no longer pay. The calculation is simple once you know a few figures and the repayment mortgage formulas.

The result helps you decide whether to overpay, how much to overpay and which option to choose between reducing the payment and reducing the term.

Before you start, find this information on your annual mortgage statement or online banking: the outstanding balance (capital still owed, not the sum of remaining payments), the interest rate currently applied, the number of payments left and their frequency (usually monthly), and the extra amount you want to pay.

Also check your mortgage offer for an early repayment charge, an annual overpayment allowance (often a percentage of the balance) and any minimum overpayment amount.

Step 1: work out the current payment and future interest

Convert the annual rate into a monthly rate: i = rate / 12. At 3% the monthly rate is 0.25%. The payment is P = B × i / (1 − (1 + i)^−n). With a £150,000 balance and 240 payments the payment is about £831.90. Future interest is P × n − B, about £49,650.

Step 2: subtract the overpayment

The new balance is B − overpayment. With a £20,000 overpayment the balance falls to £130,000. From now on monthly interest is calculated on the new balance.

Step 3a: recalculate the payment (same term)

Apply the payment formula again with the new balance and the same 240 payments: the payment falls to about £721. Future interest becomes about £43,030, a saving of about £6,600.

Step 3b: recalculate the term (same payment)

Keep the £831.90 payment and work out the months needed with n = −ln(1 − B × i / P) / ln(1 + i). The result is about 199 months: the mortgage ends about 41 months early. Future interest falls to about £34,560, a saving of about £15,100.

Step 4: compare the results

The interest saving is the difference between future interest without the overpayment and after it. Compare both options against your monthly budget, cash reserve and other financial goals.

You can do these calculations by hand, but an online calculator removes errors and lets you try many scenarios in seconds. With the SmashMyRate calculator you just move four sliders: outstanding balance, interest rate, remaining years and extra sum.

Then choose "Reduce term" or "Reduce payment": the calculator instantly shows the interest saved, the new payment or number of months and a chart comparing before and after. You can change currency and language without affecting the calculations.

Tips for reliable estimates

Use the balance as of your latest payment, not the original loan amount. Enter the interest rate actually charged, not the APRC, which also includes fees. For variable or tracker mortgages, try several rate scenarios, because the real saving will depend on future rates.

The most common mistake is confusing the outstanding balance with the total of remaining payments: the latter includes future interest and gives wrong results. Another is ignoring timing, which matters a lot: the earlier you overpay, the bigger the saving.

Finally, do not drain all your savings to pay down the mortgage: keep an emergency fund and always check the new schedule with your lender before confirming. Calculator results are estimates and do not replace your lender's official figures.

Where do I find my outstanding balance?

On your annual mortgage statement, in online banking or by asking your lender for a redemption statement.

Should I use the interest rate or the APRC?

Use the interest rate to calculate payments and interest. The APRC is for comparing the total cost of different offers.

Does a calculator replace my lender's figures?

No. It gives a reliable estimate to help you decide, but the exact amount must always be confirmed by your lender.