Mortgage Overpayment Calculator

This calculator compares your mortgage on its normal schedule against the same mortgage with overpayments, so you can see the difference in pounds and in time. It works for any UK repayment mortgage.

Start with your outstanding balance, the amount you still owe today rather than the original loan. You can find this on your latest mortgage statement or in your lender's app.

Enter your interest rate as the annual rate on your current deal, and your remaining term as the number of years left to run. If you are three years into a 25 year mortgage, the remaining term is 22 years.

Add a monthly overpayment, the extra amount you would pay on top of your normal payment each month. Then, if you have a windfall such as a bonus or inheritance, add a one off lump sum. You can use one, the other, or both. The results update instantly as you type, so try a few amounts to see what fits your budget.

The calculator first works out your normal monthly payment using the standard repayment mortgage formula. In words: the monthly payment equals the balance multiplied by the monthly interest rate, divided by one minus the quantity one plus the monthly rate raised to the power of minus the number of months. The monthly rate is the annual rate divided by twelve, and the number of months is the term in years multiplied by twelve. When the rate is zero, the payment is simply the balance divided by the number of months.

It then runs two month by month simulations. In each month, the interest charged equals the outstanding balance multiplied by the monthly rate. The principal repaid that month is the payment minus that interest. The balance falls by the principal, and the process repeats until the balance reaches zero.

The first simulation uses your normal payment only, giving the baseline total interest and payoff time. The second simulation adds your monthly overpayment to every payment and applies any lump sum at the start, so the balance falls faster, the interest charged each month shrinks, and the mortgage clears sooner.

The interest saved is the baseline total interest minus the total interest with overpayments. The time saved is the difference in the number of months taken to clear the balance, shown as years and months. The new payoff date counts the shortened number of months forward from today.

The headline figure is the interest saved over the life of the mortgage. This is money that stays in your pocket rather than going to your lender. Because interest compounds, overpayments made earlier save more than the same amount paid later, which is why starting sooner matters.

The normal monthly payment shows your contractual payment before any overpayment, so you can sanity check it against your real mortgage statement. The time saved tells you how much earlier you would be mortgage free, and the new payoff date turns that into a calendar month and year.

The total interest comparison lays the two scenarios side by side: what you would pay in interest without overpaying, what you would pay with overpaying, and the difference between them. Use it to decide whether the overpayment amount is worth it for the saving it delivers.

Remember that these figures assume a single fixed rate for the whole remaining term and that overpayments reduce the term rather than the monthly payment. Real mortgages change rate when deals end, and most lenders cap penalty free overpayments at 10 percent of the balance per year. Check your own mortgage terms, and treat the results as a planning estimate rather than financial advice.