Rule of 72 Calculator
Solve For
%
Years to Double (Rule of 72)9.0
Exact Doubling Time9.01
The Rule of 72 Calculator is a mental-math shortcut for compound growth: divide 72 by an annual rate and you get the rough number of years for money to double. This tool runs that estimate both ways, so you can find the doubling time for a given return or the return you would need to double within a set period. It also shows the exact compound result for comparison.
Formula
yearsToDouble = 72 / rate; rateToDouble = 72 / years
- rate
- Annual growth or interest rate as a percent
- years
- Number of years over which to double
- 72
- Constant that approximates ln(2) ÷ ln(1+r) × 100
How it works
- Choose whether to solve for years to double or for the required rate, using the toggle at the top.
- For years, enter an annual growth rate and the calculator divides 72 by it; for the rate, enter a target number of years and it divides 72 by that.
- When solving for years it also shows the precise doubling time from the compound-interest formula, which is very close to the Rule of 72 estimate at typical rates.
Worked example
An investment grows at 8% a year.
- Years to double = 72 ÷ 8 = 9 years.
- The exact compound doubling time is ln(2) ÷ ln(1.08) ≈ 9.01 years.
About 9 years to double at 8%, matching the exact figure of 9.01 years.
Frequently asked questions
- Why 72 and not another number?
- The exact doubling time is ln(2) divided by ln(1+rate), which scaled to percentages is near 69.3, but 72 is chosen because it divides cleanly by 2, 3, 4, 6, 8, 9, and 12, making the mental math easier while staying accurate for everyday rates.
- How accurate is the Rule of 72?
- It is very accurate for rates between roughly 6% and 10%. At much higher or lower rates the estimate drifts from the exact compound result, which is why this tool also displays the precise doubling time.
- Can I use it for inflation or any growth rate?
- Yes. The rule applies to any constant compounding rate, so you can estimate how long it takes prices to double under steady inflation or how quickly a population or balance grows.