What is compound interest?
Earning a return on your returns. It sounds too small to matter for years, and then it becomes the only thing that matters — which is exactly why most people underestimate it and start too late.
The definition
Compounding is earning a return on your returns. Simple interest pays you on your original sum forever. Compounding adds each year's gain to the pot, so next year's gain is calculated on a bigger number.
The gain rises every year without you adding a penny. That's the whole mechanism, and it sounds too small to matter — which is exactly why people underestimate it.
Why it feels like nothing, then everything
Compounding is back-loaded. The early years look almost linear and it's easy to conclude it isn't working. The growth arrives disproportionately at the end, when the pot is largest.
| £10,000 at 7% | Value | Gained that decade |
|---|---|---|
| After 10 years | £19,672 | £9,672 |
| After 20 years | £38,697 | £19,025 |
| After 30 years | £76,123 | £37,426 |
The third decade produces nearly four times what the first did, from the same money and the same rate. Nothing changed except how long it had been left alone.
The three things that decide the outcome
Time, by a wide margin
Because the effect is back-loaded, the years you cut off are always the most valuable ones. Starting ten years earlier does more than almost any improvement in returns.
Rate — and small differences aren't small
Over thirty years, 5% turns £10,000 into £43,219 and 7% turns it into £76,123. Two percentage points nearly doubles the result.
Not interrupting it
Withdrawing resets the base the compounding works from. So does selling in a panic and buying back higher.
It works against you too
Fees compound exactly the same way, just in reverse. A 1.5% annual charge isn't 1.5% off your final total — it's 1.5% shaved off your growth rate every single year, applied to a pot that should have been growing. Over decades that gap becomes an enormous share of the outcome, which is why the difference between a 0.07% tracker and a 1.5% fund matters far more than it looks.
Credit card debt at 20% compounds against you faster than any investment realistically compounds for you. That's why clearing expensive debt beats investing almost every time.
The honest caveat
What survives that caveat is the shape: time does the heavy lifting, costs matter more than they appear, and interrupting the process is expensive. See the monthly investing calculator to run your own numbers.
Compounding needs something worth holding
Oak Growth scores roughly 1,000 companies on moat, management, economics and value — so the businesses you leave alone for twenty years are ones you've actually assessed.
Explore Oak GrowthCommon questions
What is compound interest in simple terms?
It's earning a return on your returns. Each year's gain is added to your original amount, so the following year's gain is calculated on a larger sum. The growth therefore accelerates over time rather than staying flat.
Why is compound interest so powerful?
Because it is back-loaded. £10,000 at 7% gains about £9,700 in its first decade but roughly £37,400 in its third — from the same money at the same rate. The later years produce far more, which is why starting earlier matters more than almost anything else.
Does compound interest work against you?
Yes. Fees and debt compound in exactly the same way. A 1.5% annual charge reduces your growth rate every year rather than taking a one-off slice, and credit card interest around 20% compounds against you faster than most investments realistically compound for you.
Is compound interest guaranteed on investments?
No. Compound interest tables apply a constant annual rate, but real investments deliver an uneven sequence of gains and losses, and the order affects the outcome. The underlying principle holds; the smooth curve does not.
Also see: What if you invest £1,000 a month? → · What is passive investing? → · What is the best long-term investment? →