Rule of 72
Use the classic Rule of 72 to estimate how many years it takes an investment to double at a given annual return. Divide 72 by the rate and you have a fast mental-math answer.
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💡 Good to know: 72 works because doubling hinges on ln(2) ≈ 0.693; 72 is close to that and divides evenly by many common rates.
Worked example
Doubles in: 9 yrs
How it’s solved
Worked on the example above, step by step — follow along and you can do it on paper next time, no tool required.
- The Rule of 72 is a mental-math shortcut: divide 72 by the annual return percent to estimate the years to double.
- 72 ÷ rate
72 ÷ 8 = 9 yrs
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Inputs
- Rate %/yr
8
Frequently asked
How accurate is the Rule of 72?
It's a close approximation for rates between roughly 6% and 10%; at extreme rates the true doubling time drifts a little.
What doubles in 9 years?
An 8% annual return: 72 ÷ 8 = 9 years.