How is my monthly mortgage repayment actually calculated?

A repayment mortgage (the standard product for the vast majority of UK homeowners) splits every monthly payment between paying down the capital you borrowed and paying interest on what's left outstanding. Early in the term, most of your payment is interest; by the final years, it's mostly capital. The calculator uses the standard capital-repayment amortisation formula, which takes your loan amount, your interest rate and your term in years and works out a fixed monthly figure that clears the whole balance, interest included, by the end of the term.

Take the default example on this page: a £250,000 loan over 25 years at 4.57%. That works out to a monthly repayment of around £1,400, with roughly £169,900 of that made up of interest by the time the mortgage is cleared.

So if two lenders quote you similar headline rates but different terms, the monthly figure (not the rate on its own) is what actually tells you whether the deal fits your budget.

Why does a longer term lower my payment but cost me more overall?

Stretching a mortgage from 25 years to 30 or 35 years reduces your monthly outgoing because you're spreading the same capital over more payments. But you're also paying interest on a larger outstanding balance for longer, which pushes the total interest bill up significantly (often by tens of thousands of pounds), even though the rate itself hasn't changed.

25 years vs 30 years on £250,000 at 4.57%

Over 25 years: about £1,400 a month, roughly £169,900 in total interest. Over 30 years: about £1,277 a month, roughly £209,800 in total interest. The monthly saving is £123, but it costs almost £40,000 more in interest over the life of the loan.

If you can comfortably afford the higher monthly payment on a shorter term, the mortgage overpayment calculator is worth running too, since even modest overpayments on a 30-year term can claw back a large chunk of that interest gap.

A longer term is a genuine tool for short-term affordability, but it comes at a real cost over the loan's lifetime, so treat it as a temporary measure to revisit at your next remortgage rather than a fixture.

Why is the total interest figure only illustrative, not a genuine prediction?

This calculator assumes one fixed interest rate for the entire term you enter, because that's the only way to show a clean amortisation schedule. In reality, a fixed-rate deal in the UK typically only fixes your rate for 2, 3 or 5 years, not for the whole 25- or 30-year mortgage. When that fixed period ends, you'll remortgage or roll onto your lender's standard variable rate, and the rate you get next time could be higher or lower than today's (Bank of England, 2026).

That means the total interest figure this tool shows you (say, £169,900 over 25 years) is a useful benchmark for comparing two offers side by side today, but it is not what you will actually pay over the full 25 years, because your rate will almost certainly change more than once before the mortgage is cleared. Our remortgage prep guide covers what to check before each fixed period ends.

Use the total-interest figure to compare deals against each other now, not as a forecast of your lifetime mortgage cost.

What this means for you

If you're comparing two mortgage offers with the same rate but different terms, run both through this calculator before you decide. The maths points toward taking the shortest term you can comfortably afford on your current income, because every extra year you add costs you disproportionately more in interest than it saves you in monthly cash flow. If you're not sure what you can stretch to without over-committing, check the figures against our affordability calculator before you commit to a term.

Most people who run these numbers end up choosing a term that's shorter than the maximum a lender will offer them, once they see the actual interest cost in pounds rather than as a percentage. If you're in this position (deciding between a lower monthly payment now and a materially cheaper mortgage overall), treat the term length as the lever you control most directly, since the rate itself is largely out of your hands.