Rule of 72 Calculator Guide: Doubling Time, Accuracy & Limits
Divide 72 by a constant annual growth rate to approximate doubling time—fast mental math with known bias at extreme rates.
Rule of 72 Calculator Guide: Doubling Time, Accuracy & Limits
Updated May 2026 · ~8 min read
The Rule of 72 is a mental approximation for doubling time under a positive constant rate. It does not model volatile returns, changing rates, contributions, withdrawals, taxes, fees, inflation, or losses, and it is not a forecast.
When the rule of 72 is handy
- Back-of-envelope planning: you translate quoted APR-style rates into doubling horizons during conversations.
- Inflation intuition: you compare how fast purchasing power halves when real rates swing.
- Teaching exponentials: learners feel why small rate differences compound into large wealth gaps.
- Not for precision portfolios: use spreadsheets when contributions, taxes, and varying returns matter.
The formula
Approximate doubling years = 72 ÷ annual rate in percent
Compare with ln(2)/ln(1+r) and use only compatible rate conventions. Accuracy deteriorates at extreme rates.
Approximation versus exact constant-rate arithmetic
A market return assumption is not a coupon and cannot guarantee a doubling date.
Common mistakes
- Applying the rule to variable market returns as a forecast.
- Ignoring taxes, fees, inflation, or losses.
- Using zero or negative rates.
- Mixing nominal and effective rates.
Try the calculator
Use the interactive calculator to plug in your numbers and see results instantly—without redoing the math by hand.
Open rule of 72 calculator →FAQ
Does Rule of 72 guarantee doubling?
No.
Why 72?
It is convenient for mental arithmetic.
Is it exact?
No. It is an approximation.
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Educational Disclaimer
This article is for educational and informational purposes only and should not be considered investment, financial, tax, or legal advice. Market information may change over time, and readers should verify important details independently before making financial decisions.