How compound interest quietly does the heavy lifting
Published 1 July 2026
The maths of compound interest is not difficult. What catches people off guard is how little it appears to do in the first few years, and how much it does in the last ones.
Simple interest pays a return on the original sum, year after year. Compound interest pays a return on the original sum plus everything the account has earned so far. That distinction sounds minor. Stretched over decades it becomes the difference between a modest pot and a genuinely meaningful one.
What the numbers actually look like
Take someone who puts £200 a month into a Stocks and Shares ISA from age 25, at a 5% annual return. After 30 years they have paid in £72,000. The account holds around £166,000. The extra £94,000 came from interest compounding on itself, and it is more than the total they ever contributed.
Now take someone who waits until 35 to start the same plan. They pay in £48,000 over 20 years and reach roughly £82,000 by 55. Starting a decade later costs about £84,000 in final value, despite contributing only £24,000 less. The missing £60,000 was not effort or extra contributions. It was time.
| Starting at 25 | Starting at 35 | |
|---|---|---|
| Years invested | 30 | 20 |
| Monthly contribution | £200 | £200 |
| Total paid in | £72,000 | £48,000 |
| Approximate value at 55 | £166,000 | £82,000 |
Both examples assume a steady 5% a year, which real markets never deliver smoothly. The point is the shape of the outcome, not the precision of the figures.
The comparison also illustrates something counterintuitive. The early decades of a compound investment can look almost boring. Between 25 and 35, that £200-a-month investor watches the account creep from zero to around £31,000. But that £31,000 then keeps compounding regardless, and it is precisely that early base doing the heavy lifting in the later years.
The Rule of 72
There is a useful mental shortcut: divide 72 by the annual interest rate and you get roughly how many years it takes a lump sum to double. At 5%, that is about 14 years. At 7%, just over ten. At 2%, closer to 36.
It only works for lump sums sitting untouched, not for regular contributions. But it is a handy sense-check. If someone promises to double your money in five years, the implied rate is around 14.4%. That should prompt some questions about what risk is attached.
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What quietly eats into the return
Three things drag on real-world compound growth that headline rates tend to gloss over.
Inflation. If an account returns 5% and inflation is running at 3%, the real increase in purchasing power is closer to 2%. Compound interest is not cancelled by inflation, but the real return is considerably lower than the nominal figure. A pot that grows from £10,000 to £16,000 over ten years has not meaningfully grown by 60% if prices have also risen substantially over the same period.
Charges. A fund or platform taking 0.75% a year sounds like a rounding error. It is not. Charges compound the same way returns do, except in the wrong direction. Over thirty years, a 1% annual fee versus a 0.1% annual fee can cost more than £30,000 on typical figures. Low-cost index funds exist because this drag is real and almost invisible in the short term.
Tax. Savings interest outside an ISA wrapper is taxable above the Personal Savings Allowance (£1,000 for basic-rate taxpayers, £500 for higher-rate, in 2026/27). Inside an ISA, growth is free of income tax and capital gains tax. The annual ISA allowance is £20,000, and for anyone putting money away for the long term, filling it before looking at taxable accounts is usually the right order of priority.
The one thing that breaks it
Compound interest concentrates its gains heavily in the later years. The last decade of a thirty-year investment typically produces more growth than the first two decades put together. Withdrawing early, or stopping contributions during a rough patch, cuts into exactly the period that does the most work.
That is not an argument for never touching the money. It is an argument for keeping a separate emergency fund, so you are never forced to raid the long-term pot because of a short-term squeeze. The two pots serve different purposes and should be treated differently.
For short-term savings, compound interest matters much less anyway. If you are saving over six months or a year, the difference between simple and compound interest amounts to a few pounds at most. It earns its reputation over years and decades, which is precisely why starting earlier matters more than starting with a larger sum.
Frequently asked questions
This guide is general information, not financial advice. All figures are illustrative; actual returns will vary and past performance is not a guide to future results.