Compound interest calculator
See what regular saving really becomes. Set one starting amount and time horizon, then compare three plans side by side. Vary the monthly contribution and growth rate to see, on one chart, what saving more or earning a better rate is worth over the years. Built for Stocks & Shares ISAs, Cash ISAs and savings accounts.
Balance over time
Nominal value
Year-by-year balance
| Year | Plan A | Plan B | Plan C |
|---|
Assumptions: compounded monthly · contributions at the start of each month · 0.2% annual charges taken off the growth rate · ISA-style tax-free growth (no income or capital gains tax) .
This is an illustration, not a guarantee.
Investment values can fall as well as rise and you may get back less
than you put in. Past returns don't predict the future.
Planning a pension?
Employer contributions and tax relief change the maths significantly.
A dedicated pension calculator is coming soon.
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OpenHow this is calculated
- We start with your starting amount (money already invested on day one), which is the same across all three plans.
- Each month we add that plan's monthly contribution at the start of the month, then grow the whole balance by one month's return.
- The growth rate you enter is the gross return. We take your annual charges off it first, then compound monthly, so an entered 5% means exactly 5% a year.
- We repeat this for every month over the number of years you choose, recording the balance at the end of each year for the chart and table.
- Total paid in is your starting amount plus every monthly contribution. Total growth is whatever the final balance is above that. It can be negative if charges outweigh the return.
- With the inflation toggle on, we discount each year's balance back to today's money, so the figure shows real spending power in today's terms.
- No tax is applied: a Stocks & Shares or Cash ISA grows free of income tax and capital gains tax.
Frequently asked questions
How does compound interest work?
You earn returns on your original money and then earn returns on those returns too. Over long periods this growth-on-growth becomes the largest part of your pot, which is why starting early matters more than the exact amount.
What growth rate should I use for a Stocks & Shares ISA?
There is no guaranteed figure, but a globally diversified equity fund has historically returned roughly 5–7% a year before charges over the long term, versus around 2–4% for a Cash ISA or savings account. Lower the rate for a more cautious projection.
How much can I pay into an ISA each year?
£20,000 across all your ISAs in the 2026/27 tax year, about £1,667 a month. The calculator still runs above that, but contributions over the allowance cannot go in an ISA.
Does compounding frequency matter?
This calculator compounds monthly, so an entered rate of 5% means exactly 5% a year. Daily versus monthly versus annual compounding makes only a small difference at typical rates. The contribution amount and time horizon matter far more.
Is growth in a Stocks & Shares ISA really tax-free?
Yes. There is no income tax or capital gains tax on growth or withdrawals inside an ISA, and nothing to report to HMRC. That is why this calculator applies no tax to the result.
What is the difference between the nominal and "today's money" figures?
The nominal figure is the pound amount you will see on your statement. The inflation-adjusted figure shows what that is worth in today's spending power, the more honest gauge of whether you are really getting ahead.
The classic plain-English case for low-cost, long-term index investing. It matches the mindset of anyone modelling 30-year ISA growth.
This calculator is for general guidance only. It does not replace advice from a qualified financial adviser on your personal circumstances.
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