Divide 72 by your annual return rate to estimate how long it takes to double your money. See how the rule compares to the exact logarithmic formula and how many doublings you get over any horizon.
Enter your expected annual return (0.01–99%)
The Rule of 72 is a mental math shortcut: divide 72 by your annual return rate to estimate how many years it takes to double your money. At 8%/yr, 72 ÷ 8 = 9 years. The exact answer from logarithms is 9.006 years — remarkably close.
The rule is most accurate for rates between 6% and 10%. At very low rates (1–2%) or very high rates (50%+) the approximation drifts, but it remains useful for quick mental estimates across a wide range.
| Rate | Rule of 72 | Exact | Error |
|---|---|---|---|
| 2% | 36.0 yr | 35.0 yr | +2.9% |
| 6% | 12.0 yr | 11.9 yr | +0.9% |
| 8% | 9.0 yr | 9.0 yr | +0.1% |
| 10% | 7.2 yr | 7.3 yr | −0.9% |
| 20% | 3.6 yr | 3.8 yr | −4.9% |
69.3 is the mathematically precise constant (ln(2) × 100), but 72 has more integer factors (2, 3, 4, 6, 8, 9, 12) making mental math easier. 70 is sometimes used for rough estimates.
Yes — divide 72 by the inflation rate to estimate how long before prices double (or purchasing power halves). At 3% inflation, 72 ÷ 3 = 24 years.
The rate you entered is your assumption. Your actual XIRR may differ significantly based on when you invested and withdrew. Upload your brokerage export to find out your real money-weighted return.