Rule of 72 Calculator
Use the Rule of 72 to estimate how long it takes to double your money at a given interest rate, or what rate you need to double in a given time.
Doubling Estimate
The Rule of 72 is a quick mental math shortcut for estimating compound growth:
Years to Double = 72 / Interest Rate (%)
Or in reverse:
Required Rate = 72 / Years to Double
Examples:
- At 6% interest: 72 / 6 = 12 years to double
- At 8% interest: 72 / 8 = 9 years to double
- At 12% interest: 72 / 12 = 6 years to double
- Double in 5 years: 72 / 5 = 14.4% required rate
Why 72?
- 72 is divisible by many small numbers (2, 3, 4, 6, 8, 9, 12) making mental math easy
- It is most accurate for rates between 6–10%
- For lower rates, the Rule of 70 is slightly more accurate
- For higher rates, the Rule of 69.3 is more precise
Comparison to exact doubling time:
- The exact formula is:
ln(2) / ln(1 + r) ≈ 69.3 / rate - The Rule of 72 adds a small correction that improves accuracy in the 6–10% range