Compound interest calculator
See how a starting amount and a regular monthly contribution grow over time, and how much of the final balance is growth rather than what you paid in.
How this is calculated
Interest is compounded monthly at one twelfth of the annual rate, and each monthly contribution is added at the end of the month.
The starting amount grows for the full period. Each contribution only grows for the time remaining after it is paid in, which is why the early years matter more than the later ones.
Total paid in is the starting amount plus every contribution. Growth is whatever the final balance exceeds that by.
Nothing here adjusts for inflation or tax. A 4.5% return with 3% inflation is a 1.5% real return, and interest outside an ISA may be taxable.
Rates used · 2026/27
| Compounding | monthly |
| Contributions | added at the end of each month |
| Inflation | not applied |
| Tax | not applied, check your ISA allowance |
Last checked July 2026. Source: HMRC and gov.uk.
Worked example
Starting with £5,000 and adding £200 a month for 10 years at 4.5% reaches £38,075. You paid in £29,000, so £9,075 of that is growth, just under a quarter of the final balance.
Common questions
Why does the order of the years matter?
Money paid in early compounds for longer. The same total contributed over ten years produces a noticeably larger balance if more of it goes in at the start.
Should I use a stock market return here?
You can, but be careful. Market returns are not a steady annual rate, and a sequence of bad early years produces a very different outcome from the same average delivered smoothly.
Does this account for inflation?
No. To see the result in today's money, subtract your inflation assumption from the rate before entering it.
Is the interest taxable?
It can be. Savings interest above your personal savings allowance is taxable outside an ISA, and this calculator does not deduct that.