Built and fact-checked by the DocNectar team — see our editorial standards
Key Features
Instant Calculation
Get accurate results in real time with our optimized algorithm.
Mobile Friendly
Fully responsive design. Works on all devices & screen sizes.
Privacy Focused
Your data stays on your device. We don't store any inputs.
100% Free
No hidden costs. This tool is completely free forever.
The Rule of 72 is a quick mental-math shortcut for estimating how many years it takes an investment to double in value at a fixed annual rate of return: just divide 72 by the rate. It's remarkably close to the exact answer for typical interest rates, which is why it has stayed popular for over a century.
This calculator gives you both the quick Rule of 72 estimate and the exact figure (calculated with logarithms), so you can see how close the shortcut really gets.
How it works
Enter an annual interest rate. The tool divides 72 by that rate for the quick estimate, and separately calculates the exact number of years using the formula ln(2) / ln(1 + rate).
- Enter annual interest rate (%).
- Click Calculate to see your results.
Examples
Doubling time at 6% annual return
At a 6% annual return, the Rule of 72 estimates 12 years to double, versus an exact figure of about 11.9 years.
Who should use it
- Quickly comparing how different interest rates affect investment doubling time.
- Estimating how fast inflation erodes purchasing power at a given rate.
Industry applications
- Personal finance and investing
- Financial education and advising
Advantages
- Extremely fast mental-math shortcut for comparing investment growth rates.
- Shows the exact logarithmic answer alongside the estimate for comparison.
Limitations
- Estimate accuracy degrades at very high or very low interest rates.
- Only applies to compound growth, not simple interest.
Common mistakes to avoid
- Applying the Rule of 72 to simple (non-compounding) interest, where it doesn't apply.
- Treating the quick estimate as exact for very high or very low interest rates, where the gap from the real figure widens.
Best practices
- Use the Rule of 72 for quick mental comparisons, and the exact figure shown here when precision matters.
Tips
- For rates far from the 6-10% sweet spot, use the exact figure shown alongside the Rule of 72 estimate rather than the shortcut alone.