Compounding is what happens when your investment returns start earning returns of their own. In the early years it looks slow. Over decades it becomes the single biggest driver of most investors' wealth.
A simple example
Invest $10,000 and earn 7% a year. After one year you have $10,700. In year two you earn 7% on $10,700, not just on your original $10,000. Keep going for 30 years without adding a dollar and your $10,000 grows to about $76,000. More than $66,000 of that is growth, and most of it arrives in the later years.
Adding regular contributions
Compounding is even more powerful when you keep investing. Putting $500 a month into an investment earning 7% a year:
- After 20 years: about $260,000, from $120,000 of contributions
- After 30 years: about $610,000, from $180,000 of contributions
The extra 10 years add $60,000 of contributions but roughly $350,000 of value. That's why starting early matters more than almost anything else. Run your own numbers in the compound interest calculator.
These examples assume a steady 7% return to keep the math simple. Real stock returns vary a lot from year to year, including losing years, and past returns don't guarantee future ones.
The Rule of 72
A quick shortcut: divide 72 by your annual return to estimate how many years it takes to double your money. At 7%, money doubles roughly every 10 years. At 10%, about every 7 years. At 4%, about every 18 years.
What helps compounding
- Time: the longer your money stays invested, the more compounding does for you.
- Reinvesting: reinvest dividends rather than spending them. See the dividend calculator to compare.
- Low fees: a 1% annual fee compounds against you, and over 30 years it can consume a large share of your final balance.
- Taxes: tax-advantaged accounts let growth compound without yearly tax drag. See Roth vs. traditional IRA.
- Staying invested: selling during downturns interrupts compounding at the worst time.
Measuring your own growth
To see the annual rate an investment has actually compounded at, use the CAGR calculator. For planning further ahead, the retirement calculator shows whether you're on track.
This article is educational and not investment advice.
Frequently asked questions
What is the Rule of 72?
A shortcut for estimating how long it takes money to double: divide 72 by the annual return. At 8% a year, money doubles in roughly nine years.
How often does compound interest compound in the stock market?
Stocks don't pay a fixed interest rate. Compounding happens as share prices grow and reinvested dividends buy more shares, so returns vary from year to year.
Why does starting early matter so much?
Because growth builds on past growth. The final decades of a long investment period often add more than the early decades, so extra years in the market can matter more than larger contributions.
StockEngine articles are for education only and are not investment advice. Some links may be affiliate links.