Rule of 72 Calculator
Estimate how many years it takes an investment to double at a given rate.
The Rule of 72 is the most useful piece of mental math in finance: divide 72 by an annual return rate and you get the approximate number of years for money to double. At 8% a year, money doubles in about 9 years; at 12%, in 6. It's a back-of-the-envelope shortcut that reveals the startling power — and cost — of compounding without a calculator.
Enter a rate to see the doubling time, or flip it: enter a target number of years to find the rate you'd need. The rule is an approximation (exact for continuous compounding, close for typical rates), but it's remarkably accurate for the 6–10% range most investments live in.
How to use Rule of 72 Calculator
- 1
Enter an annual rate
Your expected return or interest rate.
- 2
See the doubling time
72 ÷ rate ≈ years to double.
- 3
Or reverse it
Enter years to find the rate needed.
Features
- Doubling time from any rate
- Reverse mode: rate needed for a target
- Instant mental-math shortcut
Frequently asked questions
Common mistakes to avoid
- Treating the rule as exact — it's an approximation, best for the 6–10% range.
- Entering a decimal (0.08) instead of a percentage (8).
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