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:
- Loan amount (principal) — the most direct lever. Every £10,000 extra adds roughly £55–60/month on a 25-year term at 5%.
- Interest rate — moving from 4% to 6% on a £200,000 loan adds around £230/month.
- Loan term — a longer term lowers your monthly payment but significantly increases total interest paid.
- Down payment — a larger deposit reduces the loan amount and may eliminate the need for private mortgage insurance (PMI).
A 15-year term at 5% on a £200,000 loan costs £1,582/month but saves over £60,000 in total interest compared to a 25-year term.
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.
- 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. Common planning questions it 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?
- How does overpaying by £100/month affect the total interest paid?
All of these take under a minute to model, and the answers often shift what feels affordable before you make an offer.