Compound interest means you earn a return not only on the money you put in, but also on the returns that money has already earned. Each year’s gain becomes part of the base for next year’s gain. The effect is small at first and large later, which is why time is the most valuable ingredient in investing.
A simple example
Put $1,000 in an account earning 7% a year and leave everything in it:
- After 1 year: about $1,070
- After 10 years: about $1,967
- After 20 years: about $3,870
- After 30 years: about $7,612
The first ten years add less than $1,000. The last ten add more than $3,700, with no new money. With simple interest (7% of the original $1,000 only) you would have $3,100 after 30 years.
Note that 7% is an illustration, not a promise. Stock returns vary a lot from year to year; savings accounts and bonds usually pay less.
The rule of 72
Divide 72 by the annual return to estimate how many years it takes to double your money. At 6%, roughly 12 years. At 9%, roughly 8 years. At 2%, 36 years. It is a quick mental check, not an exact formula.
Why starting early beats investing more
Imagine two people who each earn 7% a year:
- Alex invests $200 a month from age 25 to 35, then stops. Total contributed: $24,000.
- Sam invests $200 a month from 35 to 65. Total contributed: $72,000.
At 65, Alex ends up with roughly as much as Sam, despite putting in a third of the money, because Alex’s early dollars had 30 extra years to compound. The exact numbers depend on returns, but the lesson holds: the first years count the most. If you are starting now, read how to start investing with little money.
Compounding also works against you
- Credit card debt compounds too, often above 20% a year. That is why paying off expensive debt usually comes first.
- Fees compound in reverse. A 1% annual fee looks small, but over 30 years it can take a large share of your final balance. See how to minimize investment fees.
- Inflation quietly reduces what your compounded money can buy.
How to let compounding work
- Reinvest dividends and interest instead of spending them.
- Contribute regularly, even small amounts.
- Avoid withdrawing early, which resets the clock (and may trigger taxes or penalties in retirement accounts).
- Keep costs low.
You can test your own numbers with the free compound interest calculator on Investor.gov, run by the SEC.
