⚡ TL;DR
A standard repayment mortgage payment is set so that the loan is paid off exactly on the final scheduled date, assuming the rate never changes. The math is one line. What the bank doesn't show you is how much of every payment is interest in the early years, how the rate, term, and overpayments interact, and what happens when the fixed period ends.
What a "mortgage payment" actually is
For a fixed-rate, capital-and-interest (repayment) mortgage, the monthly payment is set by the standard amortisation formula:
P × r × (1+r)^n / ((1+r)^n − 1)
Where P is the loan principal, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments. That formula gives the same payment every month for the life of the deal, and it's the formula every bank's calculator uses.
The point of that formula isn't to be clever. It's the unique payment amount that exactly amortises the loan to zero at the end of the term, assuming the rate never changes.
What the bank's calculator doesn't tell you
Three things, mostly.
1. The interest front-loads the payment
Because the loan balance is highest in month 1, interest is highest in month 1. The first payment of a typical 25-year mortgage is something like 65–80% interest. By year 15, that same payment is something like 30% interest. The balance isn't a straight line; it's a curve that flattens out.
That's why the loan balance in year 5 can feel like it has barely moved. It has. The bank is just charging you interest on a much larger number than you owe today.
2. The "headline rate" is rarely the rate you pay
Almost no UK or US mortgage is fixed for the full 25–30 year term. A typical UK deal is 2, 3, or 5 years fixed, then reverts to the lender's standard variable rate (SVR), then you re-mortgage. So the payment the calculator shows you is the payment during the fix, not the payment over the life of the loan.
Two ways to think about that:
- Worst case: you stay on SVR for the rest of the term. SVR is usually higher than any fixed deal you've ever seen.
- Realistic case: you re-fix every 2–5 years. Your payment fluctuates. Your total interest paid is higher than the calculator implied.
3. Overpayments cut the term, not the payment
If you overpay by £100/month on a 25-year mortgage at 5%, you don't reduce your monthly payment — the bank reduces the term instead, because you've shortened the time needed to clear the balance. The interest saved is close to 40% of the overpayment over the remaining term, depending on when you start.
Most lenders let you overpay by 10% of the balance per year without penalty. After that, an early-repayment charge (ERC) kicks in during the fix.
The honest way to use a mortgage calculator
Open a calculator and put in three things:
- The amount you're actually borrowing (purchase price minus deposit, plus any fees rolled in).
- The real rate, not the "headline" rate. If the deal is 5 years fixed at 4.5%, and you expect to re-fix at 5.5% after that, model the loan at 4.5% for 5 years and 5.5% for 20. The number will be larger than the bank's.
- The term you actually want to be done by, not the maximum the lender will give you.
Then ask: If I overpaid by £200/month, what does the term shorten to? That single number often decides whether the overpayment is worth it.
What a "mortgage" actually costs over its life
On a £250,000 loan at 4.5% over 25 years, the monthly payment is around £1,389. Over 25 years, you pay £416,556 — so £166,556 of that is interest. On a 30-year term at the same rate, the payment drops to about £1,267, but the total interest paid rises to about £206,000. Stretching the term by 5 years costs you £40,000 of extra interest for a £122/month saving.
That's the part the bank's calculator never shows you in the summary box. It's the most important number on the page.
What a good mortgage calculator should tell you
- Monthly payment for the fix, then for the reversion rate, then for the re-fix, all the way to the end of the term.
- Total interest paid, not just the monthly number.
- The effect of a fixed monthly overpayment on the term and the total interest.
- The break-even point of any fee you pay (product fee, valuation fee, legal fee) versus the rate saving.
- What happens to the payment if rates go up by 1, 2, or 3 points at the end of the fix.
What this site doesn't do
This isn't financial advice. The math is right; the inputs are yours. The right mortgage for a 28-year-old buying a first flat in Leeds and the right mortgage for a 45-year-old remortgaging a family home in Kent are different problems, and a calculator can't tell you which deal is "right". What it can do is keep you honest about what you're signing up for.