Compound Interest Calculator
See how compounding works mathematically. Adjust the numbers to explore the concept — this is an educational illustration, not a prediction or forecast of what you might achieve.
Adjust the numbers to explore how compounding works mathematically. This is an illustration — not a prediction or forecast.
This calculator is for illustration only. It assumes a constant rate of return every year — real markets go up and down, sometimes sharply. Capital at risk. The value of investments can go down as well as up and you may get back less than you invest. Past performance is not a reliable indicator of future results.
Your calculation
Adjust the sliders — results are illustrative only.
Some people use 7–8% as a rough illustration when thinking about long time horizons, but real markets go up and down — sometimes sharply — and returns are never guaranteed. Past performance is not a reliable indicator of future results.
Illustrative value after 20 years
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at 7% illustrative annual return (not guaranteed)
You put in
—
£100/month × 240 months
Market grew
—
That's the power of compounding
How it could grow year by year
Illustration only. The chart assumes a constant annual return — real markets go up and down.
Market growth Your contributions
Year labels shown every 5 years. Hover bars for detail.
The Buy Less Crap Moment
One less coffee a day.
A daily £3.50 coffee is roughly £105/month. If I skipped one a day and invested that instead, here's how the numbers could play out — this is an illustration, not a prediction.
10 years
£19,209
illustrative example only
20 years
£61,847
illustrative example only
30 years
£156,488
illustrative example only
One small habit change could make a significant difference over decades — though the outcome depends on investment performance, which is not guaranteed.
Illustration only. Returns are not guaranteed. Investing involves risk and values can fall as well as rise.
What This Tells Us
Three lessons from the numbers
Time is everything
Starting early matters far more than the amount. 20 years beats 10 years more than doubling your contribution does.
Small amounts add up
£50/month may not sound like much. Over 25 years, compounding can make a meaningful difference — though this is an illustration only and returns are never guaranteed.
Growth accelerates
The curve steepens over time. The last 10 years of a 30-year run generate more growth than the first 20 combined.
Curious how I approach things?
This calculator illustrates how compounding works. If you're curious about my personal approach to investing, I share what I do — but it's not a recommendation for anyone else.
For educational and illustrative purposes only. This is not financial advice. Capital at risk. The value of investments can go down as well as up and you may get back less than you invest. Past performance is not a reliable indicator of future results. Tax treatment depends on individual circumstances and may change. Always do your own research before investing.