How UK mortgage payments are calculated

For a standard capital-and-interest mortgage you need three inputs: how much you borrow (P), the annual interest rate, and how many years you take to repay. Everything else — property price, deposit, LTV — matters because it changes those three, or the rate a lender will offer, not because the formula uses the house price directly.

If you know the price and the deposit, the loan is price minus deposit. A £312,500 purchase with a £62,500 deposit is a £250,000 loan at 80% loan-to-value. Lenders price by LTV band, so a larger deposit can cut the rate as well as the loan.

Convert the annual rate to a monthly rate r (annual ÷ 12). Convert the term to months n (years × 12). The level monthly payment is M = P × [r(1+r)^n] ÷ [(1+r)^n − 1]. If the rate were 0%, it would simply be P ÷ n.

That formula is what our mortgage calculator uses. Lenders may round to the nearest penny, charge daily interest, or adjust for the exact number of days in a month, so a Decision in Principle can differ by a few pounds. Treat a web result as a planning estimate.

Here P = 250,000, annual rate = 4.5% so r = 0.045 ÷ 12, and n = 300. This site’s calculator returns £1,389.58 a month. Over 25 years you would repay about £416,874 in total, of which about £166,874 is interest — if the rate never changed, which in real life it usually does after a two- or five-year fix.

The same loan over 30 years would cut the monthly payment and raise total interest. A higher rate at the same term does the opposite to the payment. That is why comparing quotes means changing one input at a time, not staring at a single “from £1,200 a month” headline.

Most UK residential deals are fixed for two or five years, then move to a (often higher) reversion or standard variable rate unless you remortgage. A single-rate calculator is still useful: it lets you compare two quotes on equal terms, and it lets you stress-test a higher rate by typing one in.

It is not a full cash-flow of “two years at 4.5% then 23 years at 7%”. If you need that picture, run the calculator twice and weight the years, or ask a broker for an illustration. Always read the APRC and fees on the lender’s document — those are not inside the amortisation formula.

This guide and our calculator are for repayment mortgages. An interest-only payment is simply loan × monthly rate; the capital is still there at the end unless you have a separate repayment vehicle. Interest-only is less common for ordinary residential purchases and is underwritten differently.

Arrangement fees, valuation fees, broker fees, buildings insurance, ground rent, service charges and early repayment charges sit outside the formula. Stamp duty is paid at purchase. Council Tax is paid to the local authority every month or year. Affordability checks also look at your income, other debts and living costs — passing a payment formula is not the same as being offered the loan.

A sensible next step on this site is to line the monthly repayment up with take-home pay, add a Council Tax estimate, and keep stamp duty in the cash-at-completion pot rather than in the monthly budget.