Mortgage Payoff Calculator

What this calculator does

This mortgage overpayment calculator shows what regular overpayments do to a UK repayment mortgage. Enter your outstanding balance, your interest rate, the years remaining, and how much extra you can pay each month, and it returns the interest saved and the number of months knocked off the term.

The table runs both schedules side by side — what the balance does under the contracted payment, what it does with your overpayment, and the gap between them. That gap widens faster than the money you put in, because every pound of capital cleared today cancels all the interest that pound would have accrued between now and the end of the term.

When to use it

The obvious moments are a pay rise, a bonus, the month car finance ends, or an inheritance. Less obvious but more valuable is the run-up to a remortgage: if a lump sum before you switch drops your loan-to-value into the next band, you gain both the interest saved and a better rate on the whole balance for the next deal.

It is also worth running against your annual overpayment allowance, usually ten percent of the balance. Most people overpay far less than they are permitted, so there is normally room to increase without any early repayment charge. And it answers the retirement-deadline question: try amounts until the months-saved figure clears the mortgage before you stop working.

Understanding the inputs

Current balance is the figure on your latest annual statement or in your online account, not the amount you originally borrowed. Interest rate should be your current product rate, and it is worth rerunning at your lender's standard variable rate to see how much more overpaying matters if you end up on it.

Remaining term is the years left on the mortgage, not the original term. Extra monthly payment is where to experiment: test small figures, because early pounds save disproportionately more than later ones, and test the figure that uses your full ten percent annual allowance. Before committing to a large amount, confirm your allowance and whether the lender applies overpayments to the term or the monthly payment.

How is this calculated?

Calculate standard payoff date, then recalculate with extra principal applied monthly to determine new payoff date and interest saved.

A worked example

Take a £210,000 balance at 4.75 percent with 20 years remaining. The contracted repayment is about £1,357 a month and the mortgage would cost roughly £115,700 in interest over those 20 years.

Overpay by £250 a month and the picture changes. The mortgage clears in 185 months rather than 240 — four years and seven months early — and total interest falls to about £86,300, a saving of roughly £29,400. The £250 a month is £3,000 a year, comfortably inside a ten percent allowance of £21,000 on this balance, so no early repayment charge applies. Over the fifteen and a half years it runs you pay in about £46,300 of extra capital to remove £29,400 of interest and five years of payments.

Limitations and assumptions

The calculator models monthly interest and assumes a single rate for the whole remaining term. Most UK deals fix for two to five years before reverting to a standard variable rate, so beyond your deal period the figures are indicative. It also assumes overpayments are applied to capital immediately and that your lender shortens the term rather than reducing the monthly payment.

It does not check your overpayment allowance or model an early repayment charge, and it excludes product fees, buildings insurance, and any service charge. Nor does it compare overpaying against pension contributions, which for a higher-rate taxpayer usually offer a better immediate return. Confirm the allowance with your lender, keep an accessible emergency fund, and treat the interest-saved figure as the guaranteed, tax-free return you are weighing against every other use of the money.