Mortgage Calculator (UK)
Monthly payment, total interest and a year-by-year balance table — repayment or interest-only, plus what overpaying saves. Free, no sign-up.
Your mortgage, estimated
Estimate only, from the standard repayment formula. Ignores product fees, early-repayment charges and rate changes at the end of a fixed or tracker deal. Not advice.
| Year | Balance outstanding at year end | Interest paid that year |
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The short answer
Your monthly payment on a repayment mortgage is fixed by one formula once you know the loan, the rate and the term — the same formula every UK lender uses. On interest-only, the sum is simpler still: you pay the interest each month and the amount you borrowed is still owed, in full, on the day the mortgage ends.
How the calculator works
Enter the property price and your deposit (as a cash amount or a percentage — the toggle switches which), the term and the interest rate, and pick repayment or interest-only. The tool applies the standard amortising-loan formula lenders use to set a repayment mortgage's monthly payment, then runs the year-by-year numbers so you can see how much of each year's payment is interest versus how much comes off the balance. MoneyHelper, the government-backed money guidance service, explains the same split in plain terms: with a repayment mortgage each payment covers “some of the capital you’ve borrowed as well as the interest,” so the balance falls every month; with interest-only, you “only pay the interest due on the amount you borrowed each month, and repay the capital at the end of the mortgage term” — on a £250,000 interest-only loan over 25 years, MoneyHelper's own example notes you owe the full £250,000 on day one of year 26.
Repayment vs interest-only
Repayment is the default for almost every residential mortgage today: the monthly payment is higher than the interest-only figure on the same loan, but the balance reaches zero on schedule and nothing is owed at the end. Interest-only keeps the monthly payment lower — useful cash-flow for a rental property, which is why it's the default assumption on our buy-to-let mortgage calculator — but the full loan is still due at the end of the term, and the borrower, not the lender, is responsible for having a way to repay it. The FCA's own review of interest-only borrowers found a real gap between confidence and reality: the regulator's analysis found “just under half of all interest-only borrowers are modelled as likely to have a shortfall,” despite most borrowers in the same research saying they felt confident about repaying. If you're weighing the two up on a single loan, the interest-only mortgage calculator puts the two monthly figures and the two end-of-term balances side by side.
Worked example
A £300,000 property, £60,000 deposit (20%), 25-year term, 4.5% rate, repayment:
- Loan: £240,000
- Monthly payment: roughly £1,334
- Total interest paid over 25 years: roughly £160,000 — more than half the amount borrowed, which is what a 25-year term at this sort of rate typically costs in interest, not a quirk of this example
- An extra £150 a month overpayment on the same loan cuts several years off the term and saves a real slice of that interest — the calculator above shows the exact number for your figures
What this calculator doesn't do
It doesn't price a specific lender's deal: product fees, early-repayment charges, and what happens to your rate when a fixed or tracker period ends are all missing, because they vary by product and lender. It also assumes a level rate for the whole term, which is rarely how a real UK mortgage works — most borrowers remortgage every two, three or five years onto a new rate. Use the figure here to compare scenarios (bigger deposit, shorter term, overpaying) rather than as a quote. If you're buying to let rather than to live in, the buy-to-let mortgage calculator and the stress test use the different, rent-based test lenders actually apply.
MoneyHelper: interest-only and repayment mortgages explained · MoneyHelper: mortgage calculators, read live 12 September 2026. The repayment figures above use the standard fixed-rate amortising-loan formula; not a quote from any lender.
Quick answers
On a repayment mortgage, the standard formula spreads the loan and interest into equal monthly payments over the term, so the same payment covers more interest and less capital early on, and more capital and less interest as the balance falls. On interest-only, the sum is simpler: monthly payment = loan × monthly interest rate. The calculator above runs both.
A repayment mortgage pays off both interest and capital, reaching zero owed at the end of the term. Interest-only pays the interest only, so the monthly cost is lower, but the full amount borrowed is still owed on the day the mortgage ends and has to be repaid from savings, investments or a sale.
It depends on the size of the overpayment, the rate and how early in the term you start. Because interest is charged on the outstanding balance, every pound of overpayment stops earning the lender interest for the rest of the term — which is why even a modest regular overpayment can cut years off a 25-year mortgage. Enter an amount above to see your own figure.
It calculates the same repayment or interest-only maths for any loan, so the monthly-payment and year-by-year numbers apply. But buy-to-let lenders don't decide how much to lend from this calculation — they run a rent-based affordability test instead, which is what the buy-to-let mortgage calculator and the stress test are built to show.
Product fees added to the loan, an introductory rate that changes later, a different day-count convention, and rounding all move the real figure slightly from a standard formula. Treat this as a close estimate for comparing scenarios, not your lender's exact quote.
General information, not financial advice — every figure above is an estimate from the numbers you enter, not a mortgage offer. Lenders apply their own criteria, fees and stress tests, which change. Read the full disclaimer.
Sources
The primary documents this page is built from. Links checked 5 September 2026.
- Interest only and repayment mortgages explained — MoneyHelper
- Mortgage calculators — MoneyHelper