Rule of 72 Calculator
Discover how many years it takes to double your money. Compare the classic mental Rule of 72 against the exact logarithmic formula, project multi-decade wealth milestones, and model inflation purchasing power halving.
Investment & Interest Rate
Exponential Growth: Multi-Cycle Doubling Path
Watch how small initial balances multiply across multiple 9.0-year cycles.
| Milestone | Multiple | Portfolio Value | Years Elapsed |
|---|---|---|---|
| Doubling #1 | 2x | $20,000 | ~9.0 yrs (9.01 exact) |
| Doubling #2 | 4x | $40,000 | ~18.0 yrs (18.01 exact) |
| Doubling #3 | 8x | $80,000 | ~27.0 yrs (27.02 exact) |
| Doubling #4 | 16x | $160,000 | ~36.0 yrs (36.03 exact) |
| Doubling #5 | 32x | $320,000 | ~45.0 yrs (45.03 exact) |
The Rule of 72 for Inflation
If annual inflation averages 8%, your cash will lose 50% of its purchasing power in exactly 9.0 years.
Frequently Asked Questions
Why 72 instead of 70 or 69?
The natural logarithm of 2 is approximately 0.6931 (69.3%), which represents exact continuous compounding. However, the number 72 is chosen because it has many small integer divisors: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, and 36, making fast mental division easy without a calculator.
At what interest rates does the Rule of 72 work best?
The Rule of 72 provides the highest accuracy for returns between 6% and 10% (the historical range of standard stock market portfolios). At very low interest rates (e.g. 1% to 3%), the Rule of 70 or 69.3 is slightly more accurate.
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