Interest-Only Mortgage Calculator UK
Compare interest-only, repayment and part-and-part mortgages: the monthly cost, the total interest, the capital still owed at the end and what you would need to invest each month to clear it.
Last updated: October 2026
Your mortgage
100 for fully interest-only. Enter, say, 50 for part-and-part: half interest-only, half repayment.
For the repayment vehicle, after charges. An assumption, not a promise: returns can be lower or negative.
Interest-only monthly payment
£750.00
You would still owe £200,000 at the end of the term. Clearing it means investing about £341.45 a month at 5% a year.
Side by side over 25 years
| Interest-only | Repayment | |
|---|---|---|
| Monthly payment | £750.00 | £1,111.66 |
| Total interest | £225,000 | £133,499 |
| Capital owed at the end | £200,000 | £0 |
| Monthly investment to clear it | £341.45 | £0 |
| Payment plus investment | £1,091.45 | £1,111.66 |
Interest-only saves £361.66 a month against repayment, but costs £91,501 more in interest and leaves the full loan to repay.
How it works
On an interest-only mortgage the monthly payment is simply the loan multiplied by the annual rate, divided by 12. £200,000 at 4.5% is £200,000 × 4.5% ÷ 12 = £750.00 a month. Because none of it repays capital, the full £200,000 is still owed when the term ends, and over 25 years you pay £225,000 in interest.
A repayment (capital-and-interest) mortgage on the same terms costs £1,111.66 a month, uses the standard annuity formula with interest charged monthly, and clears the debt by the end with £133,499 of interest in total.
The repayment vehicle figure is the monthly amount you would need to invest, at the return you assume, to build up the capital by the end of the term. At 5% a year, clearing £200,000 over 25 years takes about £341.45 a month. Add that to the interest-only payment and you are paying £1,091.45, close to the repayment figure, but with investment risk on top. At 3% the investment needed rises to £450.96 a month, so the combined cost of £1,200.96 is more than simply repaying.
A part-and-part (or split) mortgage puts some of the loan on interest-only and the rest on repayment. With £200,000 at 4.5% over 25 years split 50/50, you pay £930.83 a month: £375.00 interest on the interest-only half plus £555.83 on the repayment half. At the end £100,000 is still owed, which would take about £170.73 a month invested at 5% to cover.
It can suit someone who expects a lump sum later, such as a bonus, an inheritance or a pension tax-free cash sum, but wants the certainty that part of the debt is falling every month. Use the "share on interest-only" box above to try different splits.
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Reviewed by Alexandre Bernier, LLM (UK), CFP®
Educational tool: estimates only. Not individual financial, investment, tax or legal advice. Using it does not create an adviser-client relationship. Rules and figures change; check them against current HMRC and GOV.UK guidance and speak to a qualified professional. Editorial policy →
Sources
Every rate and threshold used by this calculator is taken from the official pages below. If a figure here disagrees with one of these, the official page is right, please tell us.