UK Mortgage Overpayment Calculator

A mortgage is usually the biggest debt most people will ever have, and even small, regular overpayments can shave years off the term and save a substantial amount of interest. The tricky part is knowing exactly how much difference a given overpayment actually makes, since the effect is not always obvious from the numbers on your mortgage statement.

This calculator compares your standard mortgage against the same mortgage with a monthly overpayment added, showing exactly how much time and interest you could save.

How to use the mortgage overpayment calculator

Enter your current mortgage balance, how many years remain on your term, your interest rate, and how much extra you plan to pay each month on top of your normal payment. The calculator shows your new payoff timeline, the interest saved, and your new monthly payment including the overpayment.

Why overpaying has such a big effect

Every pound you overpay comes straight off your capital balance, and because mortgage interest is calculated on that balance, a lower balance means less interest charged every single month for the rest of the loan. Overpaying early in a mortgage tends to have the biggest effect, since there is more time left for the reduced balance to keep saving you interest month after month.

This is the same reason why a repayment mortgage is weighted heavily towards interest in the early years and towards capital later on. Any overpayment you make early effectively skips ahead through that curve, reducing the balance that all future interest is calculated on for the entire remaining term, not just for the month you made the extra payment.

Worked example

Take a mortgage with £200,000 remaining, 20 years left on the term, at an interest rate of 4.5%. Without any overpayment, the standard monthly payment is around £1,265, and the total interest paid over the remaining term comes to roughly £103,672.

Now add a £200 a month overpayment on top, taking the total monthly payment to around £1,465. The mortgage is now paid off in around 16 years instead of 20, saving 4 years off the term. Total interest paid drops to roughly £80,735, a saving of around £22,936, more than the total amount overpaid across those 16 years.

Doubling the overpayment to £400 a month on the same mortgage has an even bigger effect. The term drops to around 13 years and 4 months, saving 6 years and 8 months compared with the standard term. Total interest falls to roughly £66,284, a saving of around £37,388 against the standard £103,672. Doubling the overpayment more than doubled the time saved and the interest saved, because the reduced balance compounds its effect over an even longer stretch of the remaining term.

Reducing your term versus reducing your payment

When you overpay, most lenders default to keeping your monthly payment the same and shortening the term, exactly as shown in the examples above. Some lenders will instead let you keep the original term and reduce your monthly payment going forward. Shortening the term usually saves more interest overall, since the loan is repaid faster and interest is charged for less time in total. Reducing your monthly payment instead can still be useful if your priority is more breathing room in your monthly budget rather than paying off the mortgage sooner.

Common mistakes to avoid

The most common mistake is overpaying beyond your lender's annual allowance, typically 10% of the outstanding balance, and triggering an early repayment charge that can wipe out some or all of the interest you were trying to save. Always check your specific mortgage terms before committing to a regular overpayment plan, since allowances vary between lenders and even between different products from the same lender.

Another mistake is overpaying while carrying higher interest debt elsewhere, such as a credit card. Since credit card APRs are almost always far higher than mortgage rates, clearing that debt first usually saves you more money than directing the same amount towards your mortgage. It is also worth keeping a reasonable emergency fund untouched, since overpaid mortgage money is not easy to access again if you need it unexpectedly.

It is also easy to forget that your mortgage rate itself may not stay fixed for the whole remaining term. If you are on a fixed rate deal that ends before your mortgage is fully repaid, the figures here will need revisiting once you remortgage onto a new rate, since a higher or lower rate at that point changes how much interest an overpayment actually saves you from then on.

Related calculators

If you are still deciding on your original mortgage, our mortgage calculator covers monthly repayments and a full amortisation schedule. Before completing on a property, check our stamp duty guide and stamp duty calculator for the upfront costs involved. It is also worth checking your take-home pay to make sure any overpayment plan is genuinely affordable alongside your other outgoings, not just on paper.