How Compound Interest Quietly Builds Wealth
The math behind compounding and three habits that put it to work for you.
The one-sentence definition
Compound interest is interest paid on your interest. It is the reason money invested at 25 typically grows to roughly twice as much by age 65 as the same amount invested at 35 — even though you only delayed by ten years.
A worked example
$10,000 invested at 7% with no further contributions grows to roughly $76,000 in 30 years. Add $200 a month to it and you reach approximately $320,000. The contributions matter — and so does the time.
The 'rule of 72'
Divide 72 by the annual rate of return to estimate how long it takes money to double. At 7%, that is about 10 years. At 4%, 18 years. At 10%, just over 7. The rule is a mental shortcut, not a guarantee.
Three habits that maximise compounding
Start now, even with small amounts — time is doing more work than money. Reinvest dividends and interest rather than spending them. Keep costs low, because fees compound against you the same way returns compound for you.
Why this also works against you
Compounding does not care which direction it runs. Credit card balances at 22% APR double in roughly three years if untouched. The same force that builds wealth on the investing side destroys it on the borrowing side.
Frequently asked questions
Is the 7% return realistic?
It is roughly the long-run, inflation-adjusted average of broad stock markets. Any individual decade can be much higher or lower.
Do savings accounts compound too?
Yes, but at far lower rates. They are for short-term safety, not long-term growth.
How often should compounding happen?
Monthly compounding vs annual makes a small difference; continuous compounding makes a tiny one. Focus on rate and time, not the frequency.