Estimate how long it takes to double your money, or the return needed.
Calculated instantly in your browser.
How does the Rule of 72 work?
Years to double ≈ 72 ÷ annual return (%); rearranged, required return ≈ 72 ÷ years to double. At an 8% annual return, money doubles in about 72 ÷ 8 = 9 years. The rule is an approximation of the exact logarithmic formula and is most accurate for returns of roughly 6–10%; the Rule of 70 and 69.3 suit lower rates and continuous compounding.
Understanding your result
The rule is an approximation of the exact logarithmic formula and is most accurate for returns of roughly 6–10%. The Rule of 70 and 69.3 are alternatives that suit lower rates and continuous compounding.
Formula and method
Years to double ≈ 72 ÷ annual return (%). Rearranged: required return ≈ 72 ÷ years to double.
Worked example
At an 8% annual return, money doubles in about 72 ÷ 8 = 9 years.
How to use this tool
- Choose whether you know the return or the time.
- Enter that value.
- See the estimated doubling time or required return.
Common mistakes to avoid
- Treating the estimate as exact for very high or very low returns.
- Forgetting that real returns vary year to year.
About the Rule of 72 Calculator
The Rule of 72 is a fast mental-maths shortcut: divide 72 by your annual return to estimate how many years it takes to double your money — or divide 72 by a number of years to find the return you would need.
Who should use this tool
Investors and savers who want a quick feel for how compounding grows money over time.
Benefits
- Estimate doubling time in seconds.
- Or work out the return needed to double in a set time.
- Compare with the Rule of 70 and 69.3 variants.
- Private and instant.
Practical use cases
- Sense-checking an investment’s long-term growth.
- Explaining compounding simply.
- Setting a target return for a savings goal.
Frequently asked questions
Why 72?
72 is a convenient, divisible number that closely approximates the exact doubling-time formula for typical investment returns.
When should I use the Rule of 70 instead?
The Rule of 70 (and 69.3 for continuous compounding) is slightly more accurate at lower rates; 72 is easiest for mental maths in the common 6–10% range.