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A back-of-the-napkin trick: divide 72 by your return rate to estimate how many years it takes your money to double.
The Rule of 72 is an approximation. It's most accurate for returns between 6% and 10%.
At 8%, $10,000 doubles about every 9.0 years — roughly 4 doublings over 40 years.
| Annual return | Years to double | Doublings in 40 yrs |
|---|---|---|
| 4% | 18.0 | 2× |
| 6% | 12.0 | 3× |
| 8% | 9.0 | 4× |
| 10% | 7.2 | 5× |
| 12% | 6.0 | 6× |
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