Rule of 72 Calculator
Compare the quick Rule-of-72 estimate with an exact constant-rate doubling calculation.
Quick answer
Divide 72 by the annual percentage rate for a quick estimated doubling time. ToolGemini also calculates an exact annual-compounding time for comparison.
Formula and calculation
Rule-of-72 years = 72 ÷ rate. Exact annual-compounding years = ln(2) ÷ ln(1 + rate).
Worked example
At 8%, the Rule of 72 estimates nine years. The exact annually compounded result is slightly longer, showing why the rule is a mental shortcut.
Limitations
The rule assumes a constant rate and ignores taxes, fees, volatility and changing inflation. It is most useful as an approximation.
Source and methodology
Investor.gov compound-interest resources. The source supports the general concept; ToolGemini uses only the values you enter and the formula described above.
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Frequently asked questions
Why use 72?
Seventy-two is easy to divide by many common rates and gives a convenient approximation near ordinary return ranges.
Is the result exact?
No. The calculator displays an exact annual-compounding comparison alongside the shortcut.
Can this estimate inflation impact?
It can approximate how long a constant inflation rate would take to halve purchasing power, but real inflation changes.