Rule of 72 Calculator
The Rule of 72 is a quick mental-math shortcut for estimating how many years it takes for an investment to double at a fixed annual return. Enter your expected interest rate to see the approximate doubling time and compare it to the exact figure.
How it works
The Rule of 72 estimates the number of years required to double an investment by dividing 72 by the annual rate of return: Years ≈ 72 ÷ rate. For example, at 8% the estimate is 72 ÷ 8 = 9 years.
The exact answer uses logarithms: Years = ln(2) ÷ ln(1 + rate). The Rule of 72 is a close approximation, most accurate for rates between roughly 6% and 10%, and it lets you do the math in your head without a calculator.
Tips
Use the rule for quick comparisons, but rely on the exact figure for planning. The 72 shortcut slightly overestimates at very high rates and underestimates at very low rates.
Remember the rule assumes a constant, compounding return and ignores taxes, fees, and inflation, which can all extend the real time to double your purchasing power.
FAQ
Why 72 and not another number?
72 is used because it divides cleanly by many common rates (2, 3, 4, 6, 8, 9, 12) and gives results close to the exact logarithmic answer for typical interest rates.
How accurate is the Rule of 72?
It is very accurate for rates around 6–10%. For higher or lower rates, the exact formula using natural logarithms gives a more precise doubling time.
Can I use it for inflation or debt?
Yes. The same shortcut estimates how long it takes for prices to double at a given inflation rate, or for debt to double at a given interest rate.