The mortgage payment formula
The standard formula for a fixed-rate mortgage monthly payment is: M = P × [r(1+r)^n] / [(1+r)^n – 1], where M is the monthly payment, P is the principal (loan amount), r is the monthly interest rate (annual rate ÷ 12), and n is the total number of payments (loan term in years × 12).
For example, a £200,000 mortgage at 5% annual interest over 25 years gives a monthly rate of 0.05 ÷ 12 ≈ 0.004167 and 300 payments. Plugging in: M = 200,000 × [0.004167 × (1.004167)^300] / [(1.004167)^300 – 1] ≈ £1,169 per month.
What affects your payment
Four variables drive your monthly payment. The loan amount (principal) has the most direct effect — every £10,000 extra adds roughly £55–60/month on a 25-year term at 5%. The interest rate has a similarly large impact: moving from 4% to 6% on a £200,000 loan adds around £230/month.
The loan term trades monthly payment size against total interest paid. A 25-year term costs less each month than a 15-year term, but you pay significantly more interest over the life of the loan. A 15-year term at 5% on the same £200,000 loan costs £1,582/month but saves over £60,000 in total interest versus the 25-year option.
Principal and interest vs. total payment
Your calculated monthly payment covers only principal and interest (P&I). Your actual monthly housing cost will typically be higher because lenders often collect property taxes and homeowners insurance via an escrow account, and if your down payment is less than 20%, you may also pay private mortgage insurance (PMI).
In the early years of a mortgage, the majority of each payment goes to interest rather than principal — this is called amortisation. As the balance falls, each payment shifts gradually toward principal, which is why refinancing early can save more interest than refinancing later in the term.
How a mortgage calculator helps
Rather than working through the formula manually, a mortgage calculator lets you test scenarios in seconds. You can adjust the loan amount, rate, and term to see how each change affects the monthly payment and total interest, giving you a clearer picture of what you can afford before you make an offer.
Common planning questions a calculator answers: How much does a higher down payment save per month? What happens to my payment if rates rise by 0.5%? Is a 20-year term meaningfully cheaper in total interest than a 25-year term? All of these take under a minute to model.