Compound Interest Calculator (Savings and ISA Growth)
Enter what you have, what you add each month and the interest rate to see how your savings could grow.
Paid in Interest earned
| Year | Paid in | Interest | Balance |
|---|---|---|---|
| 1 | £2,200 | £79 | £2,279 |
| 2 | £3,400 | £224 | £3,624 |
| 3 | £4,600 | £437 | £5,037 |
| 4 | £5,800 | £722 | £6,522 |
| 5 | £7,000 | £1,084 | £8,084 |
| 6 | £8,200 | £1,525 | £9,725 |
| 7 | £9,400 | £2,051 | £11,451 |
| 8 | £10,600 | £2,665 | £13,265 |
| 9 | £11,800 | £3,371 | £15,171 |
| 10 | £13,000 | £4,175 | £17,175 |
Assumes a fixed rate, interest added monthly and deposits made at the end of each month. Interest on savings outside an ISA may be taxable.
How compound interest works
Compound interest means you earn interest on your interest. Each time interest is added, it joins your balance, and the next interest payment is worked out on the bigger amount. Over long periods this snowball effect becomes large.
Final amount = starting amount × (1 + rate ÷ times a year)times a year × years + the growth on your monthly deposits
What £100 a month could grow to
| Interest rate | 10 years | 20 years | 30 years |
|---|---|---|---|
| 3% | £13,974 | £32,830 | £58,274 |
| 5% | £15,528 | £41,103 | £83,226 |
| 7% | £17,308 | £52,093 | £121,997 |
Over 30 years you would pay in just £36,000, so most of the final amount at higher rates comes from interest. Starting early matters more than saving a little extra later.
Savings, ISAs and tax
Interest in an ISA is tax-free, and you can put up to £20,000 a year across your ISAs in 2026/27. Outside an ISA, most people can earn some interest tax-free through the Personal Savings Allowance (£1,000 for basic rate taxpayers, £500 for higher rate taxpayers). Use our savings interest tax calculator to check whether you owe tax.
The rule of 72
A quick way to estimate doubling time: divide 72 by the interest rate. At 6%, money roughly doubles every 12 years (72 ÷ 6).
What this calculator assumes
A fixed interest rate for the whole period and deposits made at the end of each month. Real savings and investment returns change, and stock market investments can fall as well as rise. This is not financial advice.
Sources: GOV.UK: Individual Savings Accounts (ISAs). Figures last checked on 8 October 2026.
Frequently asked questions
How much will £10,000 grow in 10 years?
At 5% interest added monthly, £10,000 grows to about £16,470 after 10 years without adding anything.
What is the difference between simple and compound interest?
Simple interest is paid only on your original amount. Compound interest is paid on your original amount plus the interest already added, so it grows faster.
Does it matter how often interest is added?
A little. The more often interest is added, the more you earn, but the difference between monthly and yearly is small compared with the effect of the rate and the time.
Is interest on savings taxed?
Interest in an ISA is tax-free. Outside an ISA you can earn a certain amount tax-free each year depending on your income, and anything above that is taxed.
How do I account for inflation?
Enter your expected inflation rate in the optional field. The calculator then shows what the final amount would be worth in today’s money.
More from UK Answers
Figures are for the 2026/27 tax year from official sources and are estimates for guidance only, not financial or legal advice. Printed from ukanswers.co.uk.