Interest Only Mortgage Calculator
An interest-only mortgage means your monthly payments cover only the interest charged on the loan. The amount you borrowed stays the same for the interest-only period, so the balance does not go down while you pay this way. This calculator shows what your monthly payment would be and how much interest you would pay over that period.
Enter the loan amount, the interest rate (APR) and, if you want the total interest figure, the number of years of the interest-only period. The results update so you can compare different loan amounts, rates and terms before you speak to a lender.
How to use the interest only mortgage calculator
- Enter the loan amount in pounds (£) in the first box.
- Enter the interest rate (APR) as a percentage, for example 6.5.
- Enter the interest-only period in years if you want to see the total interest paid over that time. This box is optional.
- Read the monthly payment (interest only) shown in the results. This is the amount of interest you would pay each month.
- Read the interest over the period to see the total interest paid across the interest-only years.
- Change one figure at a time to see how the monthly payment and total interest respond.
The formula
- Monthly payment = loan amount × annual rate ÷ 100 ÷ 12
- Interest over the period = monthly payment × years × 12
- The balance does not go down while you pay interest only
Worked example
The figures below are made up to show the maths. They are not current prices, rates or product figures.
| Loan amount | £300,000.00 |
|---|---|
| Interest rate (APR) | 6.5% |
| Interest-only period | 10 years |
- Monthly payment (interest only) £1,625.00
- Interest over the period £195,000.00
Tips for an accurate answer
- Use the exact rate quoted by your lender, including any product fees you plan to add to the loan, as small differences in rate change the result.
- Remember that the balance does not reduce during the interest-only period, so the full loan amount is still owed at the end.
- Check that the interest-only period you enter matches the term offered to you, since lenders set their own limits.
- Treat the results as a guide to the maths only and confirm the final figures with your lender or bank before making any decision.
- Think about how you will repay the full loan amount at the end of the interest-only period, and check this with your lender.
Questions people ask
How do I use an interest only mortgage calculator in the UK?
Enter the loan amount in pounds, the interest rate (APR) as a percentage and, optionally, the interest-only period in years. The calculator shows your monthly interest-only payment and the total interest over the period.
How much interest would I pay over 10 years on an interest-only mortgage?
Enter your loan amount, rate and 10 years as the interest-only period. The calculator shows the total interest paid over those 10 years, based on the monthly interest multiplied by the number of months.
Can this calculator show a repayment mortgage payment?
No, this calculator only works out interest-only payments, where the loan balance does not go down. Use a repayment calculator if you want to see a payment that also reduces what you owe.
Does this calculator include both principal and interest?
No, it only covers the interest part of the payment. Because the loan amount is not reduced during the interest-only period, no principal is included in the monthly figure shown.
What does this calculator work out?
It works out the monthly payment on an interest-only loan, which is the interest charged each month on the amount you borrowed. It also shows the total interest paid over the interest-only period you enter.
Does my loan balance go down with an interest-only mortgage?
No. While you pay interest only, the amount you owe stays the same. You will still need to repay the full loan amount at the end of the agreed term.
What is the interest-only period?
It is the number of years during which you pay only the interest and not any of the original loan amount. Enter it in years if you want to see the total interest over that time.
Why is the interest-only payment lower than a repayment mortgage?
Because you are not paying off any of the amount borrowed, your monthly payment covers only the interest. This is why the figure shown is lower than a payment that also reduces the balance.
Sources
This calculator uses the standard loan and interest formulas above and the numbers you type. It needs no outside figures.
An estimate from the numbers you enter, worked out with the formula shown. It is maths, not financial advice: your lender or bank has the final figures. Page checked 23 Oct 2026. Read the disclaimer