Enter the amount borrowed, rate and term to see your monthly repayment and total interest. Add monthly or one-off overpayments to see the time and interest you could save.
Results are estimates for planning. Check the figures before you make a financial or health decision.
How the Mortgage calculator works
For a repayment mortgage, the monthly payment is P × r ÷ (1 − (1 + r)^−n), where P is the amount borrowed, r is the annual rate divided by 12 as a decimal, and n is the number of months. Total interest is the monthly payment times n, less the amount borrowed.
If you enter an overpayment, the tool steps through the loan month by month. Each month it adds interest to the balance, takes off your normal payment and your extra amount, and counts the months until the balance reaches zero. A one-off overpayment reduces the starting balance. It then shows the new payoff time, time saved and interest saved.
For interest only, the monthly payment is just the balance times the monthly rate, and the full amount is still owed at the end.
The tool assumes a single fixed rate for the whole term and no fees. Many lenders limit overpayments during a fixed rate, often to 10% of the balance a year, so check yours.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Amount borrowed | 250000 |
| Interest rate | 4.5 % |
| Term | 25 years |
| Mortgage type | Repayment |
| Overpay each month | 200 |
| One-off overpayment now | 0 |
Monthly repayment: £1,389.58
| Total interest | £166,874 |
|---|---|
| Total repaid | £416,874 |
| With overpayments, paid off in | 19 years, 11 months |
| Time saved | 5 years, 1 month |
| Interest saved | £38,458 |
Tips and common mistakes
Try the same loan at slightly different rates and terms. A longer term lowers the monthly payment but raises total interest, and the difference is usually bigger than people expect.
Rates change when a fixed deal ends, so test a higher rate for the remaining term to see how well you could cope. Remember fees, insurance and stamp duty are separate costs. For more detail, see How mortgage repayments are calculated. If you are still deciding how much to borrow, try Mortgage affordability. General information, not advice.
Frequently asked questions
How are mortgage repayments calculated?
The monthly payment comes from the amount borrowed, the monthly interest rate and the number of months, using the standard repayment formula. The calculator does this for you.
How much can overpaying save?
It depends on the balance, rate and amount, so enter your own figures. The tool shows the time saved and the interest saved.
Does it handle changing rates?
No. It assumes one fixed rate for the whole term. If your rate changes, run it again with the new rate and remaining balance.
Guides that help
- How mortgage repayments are calculated
- How much mortgage can you afford?
- Stamp Duty, LBTT and LTT explained