Enter a starting amount, what you’ll add each month, an expected yearly return and how many years, and the calculator shows how much you could end up with, how much of that you paid in and how much came from growth. Use “Toggle Growth Table” to see the balance year by year.
For a savings account, use its AER. For investments such as a stocks and shares ISA or a pension, use a cautious long-term figure rather than last year’s return, and try a couple of different rates. The result is a projection, not a promise.
What compound interest actually does
Compound interest is interest earned on your interest. Each month’s growth is added to the balance, so next month’s is worked out on a slightly bigger number. Over a few years the effect is small. Over twenty or thirty it’s the main event, which is why time matters more than almost anything else you put into the calculator.
How much will £100 or £500 a month grow?
Monthly contributions only, no starting lump sum, compounded monthly. The rates are examples, not predictions.
| Monthly saving | Period | At 3% | At 5% | At 7% |
|---|---|---|---|---|
| £100 a month | 20 years (£24,000 paid in) | £32,830 | £41,103 | £52,093 |
| £500 a month | 10 years (£60,000 paid in) | £69,871 | £77,641 | £86,542 |
At 5%, £100 a month for 20 years grows to about £41,100, roughly £17,100 more than you paid in. Notice how much the rate matters over the longer period: the gap between 3% and 7% is nearly £19,300 on just £24,000 of contributions.
Three examples
Lump sum plus regular investing. £10,000 to start, £1,000 a month, 6% a year for 25 years: £737,643.66 at the end, from £310,000 paid in. More than half the final figure is growth.
A single lump sum. £50,000 left alone at 7% for 20 years becomes £201,936.94. It roughly doubles every ten years.
An optimistic case. £25,000 to start, £2,500 a month at 8% for 30 years comes to about £3,999,292 from £925,000 of contributions. A steady 8% every year for 30 years is unlikely, so read this as an illustration of how compounding works rather than a forecast.
The formula
The calculator compounds monthly and adds each contribution at the end of the month: FV = P(1 + r)n + PMT × [((1 + r)n − 1) ÷ r]. P is the starting amount, PMT the monthly contribution, r the monthly rate (the annual rate divided by 12) and n the number of months. At 6% a year, r is 0.5% and 25 years is 300 months.
Does compounding frequency matter?
A little, but less than people think. £10,000 at 5% for 10 years:
| Compounding | Value after 10 years |
|---|---|
| Annually | £16,288.95 |
| Monthly | £16,470.09 |
| Daily | £16,486.65 |
UK savings accounts quote an AER, which already includes the effect of compounding, so you can compare them on that figure alone.
A quick shortcut: the Rule of 72
Divide 72 by the yearly rate to get a rough idea of how long it takes to double your money. At 6% it’s about 12 years; at 4%, about 18. It’s handy for a quick sense-check without a calculator.
Tax on growth
Where you hold the money makes a big difference. Inside an ISA, interest, dividends and gains are all tax-free, and you can put in up to £20,000 a year; the ISA Calculator covers this. In an ordinary savings account, interest above your Personal Savings Allowance is taxable, as the Savings Calculator explains. Investments outside an ISA or pension can be liable to Capital Gains Tax. Pension contributions usually get tax relief and grow tax-free inside the pension.
Things the calculator doesn’t do
It doesn’t deduct fees, tax or inflation, and it assumes the same return every year. Platform and fund charges of even 0.5% to 1% a year add up over decades, so knock that off the rate. To see roughly what the result is worth in today’s money, take expected inflation off as well. And with investments, the value can fall as well as rise; you may get back less than you put in.
Questions people ask
How much will £100 a month be worth in 20 years?
At 5% a year compounded monthly, about £41,103 from £24,000 paid in. At 3% it’s about £32,830, and at 7% about £52,093.
How much is £500 a month for 10 years with compound interest?
At 5% a year, about £77,641 from £60,000 paid in. At 7% it’s about £86,542.
Is daily or monthly compounding better?
Daily grows very slightly faster. £10,000 at 5% for 10 years gives £16,486.65 with daily compounding and £16,470.09 with monthly.
What is the Rule of 72?
Divide 72 by the yearly rate to estimate how many years it takes to double your money. At 6%, about 12 years.
Does the calculator account for inflation?
No. Results are in future pounds. Reduce the rate by expected inflation for a rough real-terms figure.
Related: Savings Calculator · ISA Calculator · Mortgage Calculator
Estimates only, not financial advice. Investments can fall as well as rise. Last reviewed: 3 November 2026.