Compound interest is the closest thing to a superpower in personal finance. Your money earns returns, and then those returns earn returns too. Left alone long enough, the growth stops looking like a line and starts looking like a hockey stick.
This guide shows exactly how it works — with real numbers for a £300-a-month saver — and how to calculate your own scenario in seconds.
Simple vs compound interest, in one example
Suppose you invest £10,000 at 6% a year.
Simple interest pays 6% on the original £10,000 only — £600 a year, forever. After 20 years you have £10,000 plus 20 × £600 = £22,000.
Compound interest pays 6% on everything in the pot, including previous interest. Year one you earn £600. Year two you earn 6% of £10,600 = £636. The annual payout creeps up every year, and after 20 years you have £32,071 — the same rate, the same period, £10,000 more.
That gap is the entire reason people say "start early".
The formula (and why you rarely need it)
For a one-off lump sum: A = P × (1 + r)n, where P is your starting amount, r the growth rate per period, and n the number of periods.
For monthly contributions like the example below, each payment compounds for a different length of time — doing this by hand is painful. That's exactly what the compound interest calculator is for: enter a starting amount, monthly contribution and rate, and it projects the full curve year by year.
What £300 a month actually becomes
Here are the real numbers for someone saving £300 every month, with no starting lump sum, at three growth rates:
| Years | Invested | At 4% | At 6% | At 8% |
|---|---|---|---|---|
| 10 | £36,000 | £44,175 | £49,164 | £54,884 |
| 20 | £72,000 | £110,032 | £138,612 | £176,706 |
| 30 | £108,000 | £208,215 | £301,355 | £447,108 |
Three things jump out:
- The last decade does the heavy lifting. At 8%, the pot grows by £270,000 between year 20 and year 30 — more than double the growth of the first 20 years combined.
- Growth outpaces your own deposits. At 8% over 30 years, you contribute £108,000 and the market adds £339,108 on top.
- Rate matters as much as time. The 30-year difference between 6% and 8% is nearly £146,000 — from the same £108,000 of deposits.
The Rule of 72
A quick mental shortcut: divide 72 by your annual growth rate to estimate how long money takes to double. At 6%, a lump sum doubles roughly every 12 years (72 ÷ 6). At 8%, every 9 years. It's not exact, but it's remarkably close for rates between 4% and 12% — handy for sanity-checking any investment pitch that sounds too good.
Where compounding shows up in real life
Pensions. A workplace pension is compound interest with a booster: employer contributions and tax relief increase the monthly amount going in, so the curve starts even higher. US readers get the same effect from a 401(k) with an employer match — never leave matching money on the table.
ISAs and index funds. A stocks & shares ISA holds funds in a tax-free wrapper, so compounding isn't slowed down by tax on dividends or gains.
Debt — in reverse. The same maths powers credit cards and payday loans. A 39.9% APR card compounds against you: 72 ÷ 40 means the balance tries to double roughly every two years if unpaid. Compounding is a tool; it doesn't care which side you're on.
How to actually get 6–8%
Nobody guarantees returns, but long-run global stock market growth has historically landed near this range before inflation. The practical takeaways most independent sources agree on:
- Use low-cost index funds rather than stock-picking — fees compound against you just as surely as returns compound for you.
- Automate the monthly contribution so you don't rely on willpower.
- Don't panic-sell in downturns; missing the recovery destroys more compounding than the crash itself.
This site doesn't give financial advice — the numbers above are illustrations, not predictions.
Run your own numbers
Open the compound interest calculator and try your own scenario: your age, your realistic monthly amount, and 2–3 different rates to see the spread. If you're saving toward a specific goal, the loan calculator shows the same compounding from the borrower's side, and the paycheck calculator helps you find room in your budget for that first £300.