How compound interest actually works (with real numbers)
7 min read · Updated 2026-10-11 · MoneyCalc
Compound interest is the rare piece of financial maths that rewards patience more than cleverness. The mechanism is simple — you earn returns on your returns — but its consequences over decades are large enough to be genuinely surprising. It also runs in reverse on debt, which is the part people underestimate.
Simple vs compound, in one example
Simple interest pays you only on the original amount. Compound interest pays you on the original amount plus everything you have already earned. Put $10,000 in at 7%: simple interest gives you $700 a year, forever, so after 30 years you have $31,000. Compound interest gives you $76,000 over the same period, because each year's return starts earning too.
The gap widens with time. In year one the difference is nothing; by year thirty it is the majority of the balance. Compounding is not a slightly better version of simple interest, it is a different curve.
The Rule of 72
Divide 72 by your annual return and you get the approximate number of years it takes money to double. At 6% it doubles in about 12 years; at 9%, about 8 years; at 3%, about 24 years. It is a rough mental shortcut, not a precise formula, but it makes the trade-off between rate and time immediately visible.
The rule also exposes an uncomfortable truth about inflation. If prices rise at 3% a year, money sitting in a 3% account doubles in 24 years in nominal terms while buying exactly what it buys today.
Why starting early beats investing more later
Two people invest at 7%. One puts in $200 a month from age 25 to 35, then stops. The other starts at 35 and pays in $200 a month until 65. The early starter contributes a third as much money and, depending on the exact assumptions, can end up with more, because the earliest contributions have the most decades to compound.
This is the single strongest argument for starting small rather than waiting until the amount feels meaningful. The first years are worth disproportionately more than the last ones.
Compounding works against you on debt
A card balance at 22% compounds in the issuer's favour. Leave $5,000 untouched and the interest alone grows faster than most people's ability to pay it down, which is why minimum payments can stretch a balance over decades.
The same force makes early repayment unusually powerful. Paying down high-rate debt is a guaranteed return equal to the interest rate — a risk-free 'investment' that no market product can match.
Where it goes wrong: fees and frequency
Fund fees compound too, just in the wrong direction. A 1% annual fee on a portfolio returning 7% does not cost you 1% of the final balance, it costs a large slice of the growth, because the fee is charged on the compounding balance every year.
Compounding frequency matters more in marketing than in practice. An account advertised as compounding daily pays only marginally more than the same rate compounded monthly. Compare the effective annual yield rather than the headline rate.
Run your own numbers
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Common questions
Is compound interest guaranteed?
On a savings account, yes up to the deposit insurance limit. In investments, no — returns vary and can be negative in any given year, which is why long horizons are essential.
What rate should I assume for planning?
Be conservative. Many planners use 5-7% nominal for a diversified portfolio over long periods. Using a high rate to make a plan look better is a common and costly mistake.
MoneyCalc provides general information only and is not financial, investment, tax or legal advice. Figures are illustrative and depend on your own circumstances. See our full disclaimer.