Rule of 72 Calculator
Use the Rule of 72 to quickly estimate how long it takes to double your money, or what return rate you need.
Enter your expected annual return rate to see how long it takes to double your investment.
Years to Double (Rule of 72)
9.0 years
Exact Calculation
9.01 years
Difference: 0.01 years
Quick Reference
| Annual Return | Years to Double | Example |
|---|---|---|
| 2% | 36 years | Savings account |
| 4% | 18 years | Treasury bonds |
| 6% | 12 years | Conservative portfolio |
| 8% | 9 years | Balanced portfolio |
| 10% | 7.2 years | S&P 500 (historical) |
| 12% | 6 years | Aggressive growth |
How It Works
- 1Choose whether you want to calculate years to double or required interest rate.
- 2Enter your known value (either interest rate or target years).
- 3The Rule of 72 instantly estimates the unknown value.
- 4Compare the Rule of 72 estimate with the exact mathematical calculation.
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a simple mental math shortcut to estimate how long an investment will take to double. Divide 72 by your annual return rate to get the approximate years to double. For example, at 8% annual return, your money doubles in about 72 ÷ 8 = 9 years.
How accurate is the Rule of 72?
The Rule of 72 is most accurate for interest rates between 6% and 10%. For very low or very high rates, the approximation becomes less precise. This calculator shows both the Rule of 72 estimate and the exact calculation so you can compare.
Can I use the Rule of 72 for inflation?
Yes! The Rule of 72 works for any compound growth or decay. At 3% annual inflation, prices double in about 72 ÷ 3 = 24 years. This helps understand how inflation erodes purchasing power.
Why 72 and not another number?
72 is used because it's easily divisible by many numbers (2, 3, 4, 6, 8, 9, 12) making mental math easier. The mathematically 'correct' number would be about 69.3, but 72 is more practical for quick calculations.