Mortgage repayment calculator
Work out the monthly payment on a mortgage, and see what the term costs you in total interest.
Rates taken from the official source and last checked on 12 September 2026.
How the payment is worked out
A repayment mortgage is a fixed monthly amount that clears both the interest and the capital over the term. The formula spreads it evenly, so the payment stays the same while its make-up shifts: early on most of it is interest, and by the end most of it is capital.
That is why overpaying early does more than overpaying late, and why a longer term lowers the monthly cost while raising the total.
A worked example
Take a £300,000 property with a £30,000 deposit, so £270,000 borrowed over 25 years at 4.5%. The monthly payment is about £1,500. Over the full term that adds up to roughly £450,000, meaning around £180,000 of interest on top of the £270,000 borrowed.
Stretch the same loan to 30 years and the monthly payment drops by about £133, while the total interest rises by roughly £42,000. Shorter terms cost more each month and less overall.
What this does not cover
Arrangement fees, valuation fees, buildings insurance and any early repayment charges sit outside the calculation. So does what happens when a fixed period ends, which for most UK borrowers is the single biggest change to their payment.
Buying rather than remortgaging? The stamp duty calculator covers the tax side. Already borrowing and wondering about paying extra, try the overpayment calculator.
Common questions
How is the monthly payment worked out?
With the standard amortisation formula. The monthly rate is the annual rate divided by twelve, and the payment is the amount that clears the loan exactly over the number of months in the term. Early payments are mostly interest, later ones mostly capital.
Why is the total interest so much higher than I expected?
Because it accumulates over the whole term. On a 25 year mortgage at a typical rate, total interest often approaches the size of the loan itself. Shortening the term or overpaying cuts it sharply, which is what the overpayment calculator shows.
Does this account for my fixed rate ending?
No. It assumes one rate for the whole term, which keeps the maths honest and simple. Most UK deals fix for two to five years, then revert to a variable rate, so treat the figure as the cost if nothing changed.
What counts as a good loan to value?
Lenders price in bands, usually at 90%, 85%, 80%, 75% and 60%. Dropping below a band boundary tends to unlock a lower rate, so a slightly larger deposit can be worth more than it looks.