Compound Interest: Why It's Called the "Eighth Wonder"
✦ Key takeaways
- Compound interest = earning on your principal and on prior earnings.
- Time beats amount; starting early is the strongest factor.
- Rule of 72: divide 72 by the return to get years to double.
- Regular small contributions beat big irregular ones.
Einstein is often quoted: "Compound interest is the eighth wonder of the world; he who understands it, earns it; he who doesn't, pays it." The attribution is doubtful, but the idea is exactly right. So what is it?
Simple vs compound
Simple interest is calculated only on the original amount. Compound interest is calculated on the principal and on accumulated earnings. Your gains start generating gains of their own. This snowballing looks slow at first, then explodes over time.
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The power in one example
Imagine two people, 8% annual return:
| Starts at age | Saves monthly | Stops at age | Value at 60 | |
|---|---|---|---|---|
| Sara | 25 | 500 | 35 (10 years only) | ~ larger |
| Khaled | 35 | 500 | 60 (25 years) | ~ smaller |
The surprise: Sara saved for only ten years then stopped, yet may end with more than Khaled who saved for 25 years — because her money had longer to compound. Time is worth more than the amount.
The quick Rule of 72
To estimate how long money takes to double, divide 72 by the return rate. 8% → 72÷8 = 9 years to double. 6% → 12 years. A rough rule, but astonishingly handy.
The practical lesson
Don't wait until you have a big sum. Start with a small amount now and contribute regularly. Consistency and an early start beat large late contributions. The best day to start was yesterday; the second best is today.
This is general education, not investment advice. All investing carries risk and returns are not guaranteed.