Compound interest calculator
Enter a starting balance, a monthly amount and an interest rate to see the balance at the end of each year and how much of it is interest.
| Year | Paid in | Balance |
|---|---|---|
| 1 | £7,400 | £7,689 |
| 2 | £9,800 | £10,502 |
| 3 | £12,200 | £13,443 |
| 4 | £14,600 | £16,520 |
| 5 | £17,000 | £19,738 |
| 6 | £19,400 | £23,105 |
| 7 | £21,800 | £26,625 |
| 8 | £24,200 | £30,308 |
| 9 | £26,600 | £34,159 |
| 10 | £29,000 | £38,188 |
How it works
Each month your deposit is added and one month's interest (the annual rate ÷ 12) is paid on the whole balance. Over time the interest earned on earlier interest becomes a large share of the total.
Tax on savings interest
Most basic-rate taxpayers can earn £1,000 of interest a year tax-free under the Personal Savings Allowance (£500 for higher-rate taxpayers, nothing for additional-rate). Interest in a cash ISA is always tax-free, up to the £20,000 yearly ISA allowance.
Examples
| Start | Monthly | Rate | After 10 years |
|---|---|---|---|
| £1,000 | £100 | 4% | £16,265 |
| £5,000 | £200 | 4.5% | £38,188 |
| £10,000 | £0 | 5% | £16,470 |
| £0 | £500 | 3% | £70,045 |
Higher rates and longer periods matter more than the size of each deposit. Inflation reduces what the final balance will buy, so compare your rate with the Consumer Prices Index when you judge whether savings are growing in real terms.
Paying off debt instead
Compounding works against you on debt. A credit card charging 24% APR costs far more than any savings account pays, so clearing expensive debt usually beats saving the same money. Mortgage overpayments are a middle case: they save interest at your mortgage rate, tax-free, but the money is harder to get back.
Questions
What is £10,000 at 5% for 10 years?
About £16,470 with monthly compounding and no further deposits.
Is AER the same as the interest rate?
AER shows what you would earn in a year with compounding included, so it lets you compare accounts. For monthly-paying accounts it is slightly higher than the gross rate.
Does the calculator include tax?
How often is interest compounded?
No. It shows interest before tax. Most people pay no tax on savings interest because of the Personal Savings Allowance or because the money is in an ISA.
Sources
Checked 28 September 2026. OneCalcs is independent and not affiliated with HMRC or any government department.