Rule of 72
Quickly estimate how long it takes for an investment to double
Enter annual return percentage
Growth Chart
Reference Table
| Rate (%) | Rule of 72 | Actual | Error |
|---|---|---|---|
| 1% | 72.0y | 69.7y | 233.9% |
| 2% | 36.0y | 35.0y | 99.7% |
| 3% | 24.0y | 23.4y | 55.0% |
| 4% | 18.0y | 17.7y | 32.7% |
| 5% | 14.4y | 14.2y | 19.3% |
| 6% | 12.0y | 11.9y | 10.4% |
| 7% | 10.3y | 10.2y | 4.1% |
| 8% | 9.0y | 9.0y | 0.6% |
| 9% | 8.0y | 8.0y | 4.3% |
| 10% | 7.2y | 7.3y | 7.3% |
| 11% | 6.5y | 6.6y | 9.6% |
| 12% | 6.0y | 6.1y | 11.6% |
| 13% | 5.5y | 5.7y | 13.3% |
| 14% | 5.1y | 5.3y | 14.7% |
| 15% | 4.8y | 5.0y | 15.9% |
| 16% | 4.5y | 4.7y | 17.0% |
| 17% | 4.2y | 4.4y | 18.0% |
| 18% | 4.0y | 4.2y | 18.8% |
| 19% | 3.8y | 4.0y | 19.5% |
| 20% | 3.6y | 3.8y | 20.2% |
| 21% | 3.4y | 3.6y | 20.8% |
| 22% | 3.3y | 3.5y | 21.3% |
| 23% | 3.1y | 3.3y | 21.8% |
| 24% | 3.0y | 3.2y | 22.2% |
| 25% | 2.9y | 3.1y | 22.6% |
| 26% | 2.8y | 3.0y | 23.0% |
| 27% | 2.7y | 2.9y | 23.3% |
| 28% | 2.6y | 2.8y | 23.6% |
| 29% | 2.5y | 2.7y | 23.9% |
| 30% | 2.4y | 2.6y | 24.2% |
About the Rule of 72
The Rule of 72 is a simple formula: divide 72 by the annual interest rate to estimate doubling time.
It works best for interest rates between 6% and 10%.
For more precision at higher rates, use 69.3 instead of 72.
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What is this tool?
The Rule of 72 is a mental math shortcut to estimate how long an investment takes to double at a given annual rate of return. Divide 72 by the interest rate to get the approximate doubling time in years. The calculator shows both the Rule of 72 estimate and the actual doubling time for rates from 1% to 30%.
How to use
- 1
Enter your interest rate
Input the annual return rate (1%-30%) to calculate doubling time.
- 2
View the result
See the Rule of 72 estimate, actual doubling time, and a full reference table for rates 1%-30%.
- 3
View results
Review the output.
Frequently Asked Questions
Why 72 and not another number?
72 has many convenient divisors (1,2,3,4,6,8,9,12) making mental math easy. The exact formula uses ln(2)/ln(1+r), and 72 is the best integer approximation for typical interest rates between 6%-10%.
How accurate is the Rule of 72?
It is most accurate for rates between 6%-10%. For very low rates (<3%) use Rule of 70, and for very high rates (>15%) the error increases. The calculator shows both estimate and actual for comparison.
Can I use this for debt doubling?
Yes—the same math applies. A loan at 9% interest will double your debt in approximately 72/9 = 8 years if no payments are made.