Compound Growth: The Only Investing Maths That Really Matters

Every piece of sensible investing advice — start early, invest regularly, don't interrupt, ignore the noise — is really one piece of advice: let compounding work. It is the only mechanism in investing that does the heavy lifting for you, and understanding it properly changes how the whole subject feels.

What compounding actually is

Simple growth pays you on your original money. Compound growth pays you on your original money plus all the growth so far. Year one's gains earn gains in year two; year two's earn in year three. The curve starts flat and bends upward — slowly, then suddenly.

Concrete version: £10,000 growing at 7% a year becomes about £19,700 after 10 years, £38,700 after 20, and £76,100 after 30. Notice the pattern: the first decade adds under £10,000; the third adds over £37,000. Same money, same rate — the last years do the most work. That is the entire argument for starting early and for not interrupting.

The rule of 72

The back-of-envelope tool: divide 72 by your growth rate to get the doubling time. At 7%, money doubles roughly every 10 years; at 4% (cash-like), every 18. Over a 36-year working life, that is the difference between roughly three doublings and two — between 8× and 4× your money. Small rate differences compound into enormous outcome differences, which is also why fees matter so much: a 1% annual fee is not "1% of your money", it is a permanently slower doubling speed.

Compounding's enemies

• Interruptions. Selling in a downturn and "waiting for things to settle" restarts the curve from wherever you re-enter — usually higher. Our guide on market crashes covers why staying in wins.
• Withdrawals. Raiding the pot early removes not just the money but all its future doublings. £5,000 taken at 35 is potentially £38,000 not there at 65.
• Fees and tax. Both are negative compounding. Wrappers like ISAs exist precisely to take tax out of the loop.
• Waiting to start. The expensive one. A 25-year-old investing £200 a month to 65 ends with markedly more than a 35-year-old investing £300 a month to 65 — despite contributing less in total. Time in is the input nothing else substitutes for.

What this means in practice

Start with whatever you have (it is less than you think), automate monthly contributions so the decision is made once, choose broad low-cost funds (index funds are the natural compounding vehicle), reinvest all income — dividends are half the long-run return when reinvested (how dividends work) — and then commit the hardest act in investing: leaving it alone for decades. The people compounding rewards are not the cleverest; they are the least interrupted.

This is general education, not personalised financial advice. Investing involves risk, including the risk of losing money, and past performance is not a guide to future returns. Nothing here recommends any specific investment — for anything genuinely complex or high-stakes, speak to a regulated financial adviser.