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Rule of 72 Calculator
Estimate how long an investment takes to double at any annual rate.
Doubling time with the rule of 72
Rule of 72 estimate: —
Exact doubling time: —
Difference: —
Value of 1000 after 10 years: —
How to use the Rule of 72 Calculator
- Enter the annual rate of return as a percentage.
- Read the rule of 72 estimate and the exact doubling time.
- Compare the difference to see how accurate the shortcut is.
Features
- Rule of 72 doubling estimate
- Exact logarithmic doubling time
- Difference between the two answers
- Ten year growth example
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Frequently Asked Questions
Common questions about the free online rule of 72 calculator.
It is a shortcut for doubling time. Divide 72 by the annual percentage rate to estimate how
many years an investment needs to double.
It is close for rates between about four and fifteen percent. The tool shows the exact
logarithmic answer so you can compare the two.
It comes from the mathematics of compound growth, where doubling time is the natural
logarithm of two divided by the growth rate.
The estimate drifts at high rates. At twenty percent the rule gives 3.6 years, while the
exact answer is about 3.8 years.
Yes. Divide 72 by the inflation rate to estimate how long it takes for prices to double and
for money to halve in value.
The exact doubling time uses the natural logarithm of two, about 69.3. The number 72 is used
because it divides more neatly.
It is the rule of 72 estimate minus the exact doubling time, so a small number means the
shortcut was accurate for that rate.
Yes. It is free and unlimited, runs locally in your browser, and needs no account, no sign-up
and no personal details.
Last updated: October 10, 2026