How compound interest works, with the actual numbers
Saving a modest amount every month looks small at first. The shape of the curve is the whole story, and time matters more than most people expect.
Compound interest is the idea that your earnings start earning too. In the first few years it barely shows. After a few decades, it does most of the work.
The easiest way to feel it is to run the numbers. Suppose you put away $200 a month and your savings grow at a steady 7% a year. Real returns are never steady, and this is an illustration rather than a forecast, but the pattern holds.
| Time | You contributed | Balance at 7% |
|---|---|---|
| 10 years | $24,000 | about $34,600 |
| 20 years | $48,000 | about $104,200 |
| 30 years | $72,000 | about $244,000 |
Look at the last row. Of the roughly $244,000, only $72,000 came out of your pocket. The remaining $172,000 is growth, and most of it arrived in the final decade.
Why the early years matter most
The growth in any year is a percentage of everything already there. A bigger pile grows faster, which means the earliest dollars have the longest time to snowball.
A handy shortcut is the rule of 72: divide 72 by the annual growth rate to estimate how many years it takes money to double. At 7%, that is a little over ten years.
What this means in practice
- Start before you feel ready. Ten years of small contributions beats a late start with larger ones, because the early money has longer to grow.
- Keep costs low. A fee of 1% a year sounds tiny, but it comes straight out of the growth rate, and over thirty years it can take a large share of the final balance.
- Stay invested through bad years. The formula assumes you do not pull money out when prices fall.
Compound interest works in reverse on debt, too. A credit card balance charging 20% a year grows against you in exactly the same way, which is why paying off high-interest debt is often the best “return” available.
This article is general information, not personal financial advice. Returns vary and your capital can fall as well as rise.
