Guide · 4 minute read

How compound interest actually works

Compound interest is growth earning its own growth: returns get added to your pot, and the next round of returns is earned on the bigger pot. It starts slow, looks boring for years, and then quietly does most of the work.

Growth on growth, in pounds

Take a concrete case: £1,000 to start, £200 a month, 5% a year. After the first year the pot is £3,507. You paid in £3,400 of that, so growth contributed a grand £107. This is the stage where most people conclude investing is pointless.

Stay with it and the curve bends. After 10 years the pot is £32,703, of which £7,703 is growth. After 20 years it is £84,919, with £35,919 of growth. After 30 years it is £170,919: you paid in £73,000 and growth added £97,919, more than your own money.

Look at what each decade contributed. The first decade produced about £7,700 of growth, the second about £28,200, the third about £62,000. Same monthly payment, same return, wildly different output, because by decade three the 5% is being earned on a six-figure pot rather than a few thousand pounds. You can watch this curve build for your own numbers in the compound interest calculator.

Why starting early beats contributing more

Because growth compounds on time, when you start matters more than how hard you push. Compare two people, both earning 7% a year.

Person A invests £200 a month from 25 to 35, then never adds another penny. Total paid in: £24,000. Left to compound until 60, it reaches roughly £198,000.

Person B waits until 35, then invests £200 a month for the full 25 years to 60. Total paid in: £60,000, two and a half times as much. Final pot: roughly £162,000.

Person A wins with less than half the money, because every pound had 25 to 35 years to double and redouble. At a more cautious 5% the two roughly tie (about £108,000 versus £119,000), which is still the same outcome for 2.5 times less cash. Either way the lesson holds: an imperfect amount invested now beats a perfect amount invested later. This is also why employer pension contributions in your twenties and thirties punch so far above their weight; check what yours are worth with the pension contribution checker.

The rule of 72: how fast money doubles

For a quick mental shortcut, divide 72 by your annual return to get the approximate years to double. At 5%, money doubles roughly every 14 years (the precise figure with monthly compounding is closer to 13.9). At 7%, roughly every 10 years.

Doubling is where the early-start logic comes from. A pound invested at 25 gets about three doublings by 67 at 5%, so it becomes roughly £8. The same pound invested at 53 gets one doubling and becomes £2. The last doubling is always the biggest in pound terms, and you only get it by starting early enough to be there for it.

Fees compound too, against you

Everything above runs in reverse for costs. A 1% annual fee does not cost you 1%; it costs you 1% of an ever-growing pot, every year, plus all the growth that money would have earned.

Numbers: £200 a month for 30 years at 5% grows to £166,452. The identical fund returning 4% after a 1% fee grows to £138,810. That innocuous-sounding 1% took £27,642, around a sixth of the final pot. The practical takeaways are unglamorous: prefer low-cost index funds over expensive managed ones, check the charges on old pensions, and keep long-term investments inside a stocks and shares ISA or pension so tax does not compound against you as well.

None of this requires cleverness. Pick a sensible low-cost fund, automate a monthly amount, start now rather than at a rounder number or a better moment, and let the maths grind away for decades.

Common questions

What annual return should I assume?

Long-run global stock market returns have averaged roughly 5% a year above inflation, with large swings year to year. Using 5% nominal is a common middle-ground assumption; run a lower figure too so you have a cautious case.

Does compounding work on savings accounts or just investments?

Both. A savings account compounds interest the same way, just at a lower rate. At 4% money doubles roughly every 18 years; at 7% roughly every 10. The mechanism is identical, the rate decides how dramatic it gets.

Do these figures account for inflation?

No, they are nominal. To think in today's purchasing power, knock 2 to 3% off your expected return before calculating. At a 5% nominal return that is roughly 2 to 3% real, which still doubles your money's buying power in 25 to 35 years.

Does compound interest work against me on debt?

Yes, and faster. A credit card at 24% APR doubles what you owe in roughly three years if untouched (72 divided by 24). That is why expensive debt should almost always be cleared before you invest: no realistic return outruns it.

Does it matter how often interest compounds?

Less than people think. Monthly compounding at 5% gives an effective 5.12% a year versus exactly 5% for annual compounding. The rate, the fees and the number of years you stay in matter far more than the compounding frequency.

Guidance and education, not regulated financial advice.