For anyone who wants to understand the single concept that does most of the heavy lifting in long-term wealth: compound interest. This is the concept guide — what it is, the Rule of 72, and why time matters more than the amount you start with. For the worked numbers on regular monthly saving, see compound interest with monthly deposits.
Simple vs compound: the difference that matters
With simple interest, you only ever earn returns on your original deposit. With compound interest, the interest you earn is added to your balance, and that larger balance then earns interest in the next cycle. The effect is a snowball: slow and almost linear at first, then accelerating as the "interest on interest" begins to eclipse your own contributions.
The Rule of 72
A quick mental shortcut for how long it takes money to double: divide 72 by your expected annual return.
- At 4%, money doubles every 18 years (72 ÷ 4).
- At 8%, every 9 years.
- At 10%, every 7.2 years.
It is an approximation, not exact, but it makes the power of a higher rate over a long horizon immediately legible.
Why time beats capital: a worked example
Two investors, both assuming an 8% annual return:
- Sam starts at 25, invests £500 a month for 10 years (£60,000 total), then stops and leaves the money invested. By 65 the pot is roughly £945,000.
- Jo waits until 35, then invests £500 a month for 30 years (£180,000 total). By 65 the pot is roughly £745,000.
Jo contributed three times as much yet finished with around £200,000 less, because Sam's money had ten extra years to compound. The lesson is uncomfortable but clear: starting earlier with a smaller amount usually beats starting later with a larger one. Verify your own scenario with the Compound Interest Calculator.
Three things that quietly destroy compounding
- Raiding the principal. Withdrawing funds mid-course resets the compounding curve. Keep a separate sinking fund for planned spending so the investment pot is untouched.
- High fees. A fund charging 1.5% a year silently removes a large slice of long-term returns. Low-cost broad-market index funds keep more of the compounding working for you.
- Ignoring inflation. If cash earns 3% and inflation is 4%, real purchasing power falls by 1% a year. For horizons over 5 years, investments have historically delivered the higher real returns needed for compounding to work in your favour.
Assumptions and limitations
The 8% figure is an illustrative long-run average based on historical global stock-market returns; past performance does not guarantee future results, investments can fall as well as rise, and you may get back less than you put in. The Rule of 72 is an approximation. Cash rates fluctuate with the Bank of England base rate. Consider your risk tolerance and time horizon, and seek independent financial advice before allocating capital to equities. Learn how we ensure accuracy and quality in our editorial policy.
Written and maintained by Shaun da Silva, Finance Consultant. Learn how we ensure accuracy and quality in our Editorial Policy.