How to Calculate Rule of 72
What is Rule of 72?
The Rule of 72 is a simple mental math shortcut to estimate how long it takes an investment to double, or what growth rate is needed to double in a given time. Divide 72 by the annual return rate to get years to double.
Step-by-Step Guide
- 1Years to double = 72 / Annual Interest Rate (%)
- 2Rate needed to double = 72 / Number of years
- 3Works best for rates between 6% and 10% (±1 year accuracy)
- 4For continuous compounding, use 69.3 instead of 72
Worked Examples
Input
8% annual return
Result
Doubles in ~9 years (72/8)
Actual: 9.01 years
Input
Want to double in 6 years
Result
Need ~12% return (72/6)
Actual rate: 12.25%
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