How the mortgage calculator works
A repayment mortgage has a fixed monthly payment that covers the interest for that month and pays off a slice of the loan. Early on, most of each payment is interest. As the balance falls, the interest shrinks and more of your payment goes towards the capital, until the loan is cleared at the end of the term. The calculator uses the standard formula lenders use for this:
P = amount borrowed, r = yearly rate ÷ 12, n = number of monthly payments
An interest-only mortgage is simpler: you pay just the interest each month (amount × yearly rate ÷ 12) and still owe the full amount at the end.
The calculator then runs through every month of the mortgage to build the year-by-year table and add up the total interest. It assumes your rate stays the same for the whole term. In reality most people fix for two or five years and then remortgage, so use the rate you expect to pay on average.
Worked example: £200,000 over 25 years at 4.5%
- Monthly rate: 4.5% ÷ 12 = 0.375%
- Number of payments: 25 × 12 = 300
- Monthly payment: £1,111.66
- Total interest over 25 years: £133,499, so you repay £333,499 in all
In the first year you pay £8,909 of interest but only clear £4,431 of the loan, leaving £195,569. On an interest-only basis the same mortgage costs £750 a month, but after 25 years you've paid £225,000 in interest and still owe the whole £200,000.
How much do overpayments save?
Every extra pound you pay goes straight off the balance, so you're charged less interest every month after. Adding £100 a month to the example above clears the mortgage in 21 years 6 months instead of 25 years and saves £21,142 in interest. Open "Overpayments" in the calculator to try your own figures.
Before you overpay, check your deal's early repayment charges. Most fixed-rate mortgages allow overpayments of up to 10% of the balance each year without a fee. It also usually makes sense to clear more expensive debts, like credit cards, first.
Monthly payments at different rates
Here's how the monthly payment on a £200,000 repayment mortgage changes with the rate and term:
| Rate | 25 years | 30 years | 35 years |
|---|---|---|---|
| 3.5% | £1,001 | £898 | £827 |
| 4.5% | £1,112 | £1,013 | £947 |
| 5.5% | £1,228 | £1,136 | £1,074 |
A longer term lowers the monthly payment but adds a lot of interest. At 4.5%, stretching from 25 to 35 years cuts the payment by about £165 a month but adds roughly £64,000 in total interest.
Tips before you apply
- Lenders look at your income and outgoings, not just the payment. Use the mortgage affordability calculator to see how much you might be able to borrow.
- A bigger deposit usually unlocks lower rates. The best rates tend to start at 60% loan-to-value.
- Remember upfront costs: arrangement fees, valuation, solicitor's fees and stamp duty.
- Start looking for a new deal about six months before your fixed rate ends, so you don't drop onto the lender's standard variable rate.
Frequently asked questions
How much is a £200,000 mortgage per month?
Over 25 years at 4.5%, a £200,000 repayment mortgage costs £1,111.66 a month. Over 30 years at the same rate it's £1,013.37 a month.
How much is a £150,000 mortgage per month?
A £150,000 repayment mortgage costs £833.75 a month over 25 years at 4.5%. Over 20 years at 4% it's £908.97 a month.
Is it better to overpay my mortgage or save?
If your mortgage rate is higher than the after-tax interest you'd earn on savings, overpaying usually saves more. Keep an emergency fund first, and check your lender's overpayment limits.
What is the difference between repayment and interest-only?
With repayment, each monthly payment covers interest and part of the loan, so you owe nothing at the end. With interest-only you pay only the interest, so payments are lower but the full amount is still owed when the term ends.
How is mortgage interest calculated?
Most UK lenders charge interest monthly on the outstanding balance at one twelfth of the yearly rate. As you repay the loan, the interest each month gets smaller.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.