Rule of 72 Calculator
The fastest mental math shortcut in finance for compound interest.
What this calculator does
The Rule of 72 is finance's most famous rule of thumb. By dividing 72 by your expected annual return rate, you can accurately estimate how many years it will take to double your money. At 6%, it takes 12 years; at 8%, 9 years; at 12%, 6 years. This calculator supports two-way calculation: input a return rate to get years, or input your target years to reverse-calculate the required return rate. It is highly accurate for interest rates between 6% and 10%.
Formula & how it works
Years to Double ≈ 72 ÷ Annual Return Rate (%). Or reverse it: Required Return ≈ 72 ÷ Years to Double.
Worked examples
- At an 8% annual return: 72 ÷ 8 = 9 years to double. 100K becomes 200K, then 400K, then 800K, expanding 8x over 27 years.
- Inflation at 3%: 72 ÷ 3 = It takes 24 years for your money's purchasing power to be cut in half.
Frequently asked questions
- Why use the number 72 and not something else?
- It's derived from the natural log function ln(2) ≈ 0.693. 72 is used because it divides cleanly by 2, 3, 4, 6, 8, 9, and 12, making mental math easy.
- Is it entirely accurate?
- For interest rates between 6% and 10%, the error is less than 0.3 years. For extremely high rates (e.g., 20%+), a 'Rule of 69' or 'Rule of 70' is slightly more precise.