How does the Rule of 72 work?

Divide 72 by the annual percentage return to estimate the years to double. At 8%, money doubles in about 72/8 = 9 years.

When should I use 70 or 69.3 instead?

72 is easiest for mental math and is most accurate around 6–10%. The 70 and 69.3 variants are closer for lower rates and continuous compounding.

Rule of 72 Calculator

Free Rule of 72 calculator. Enter an annual return to estimate how many years it takes your money to double, using the 72, 70 and 69.3 rules, alongside the mathematically exact doubling time.

Estimate how many years it takes an investment to double.

t \approx \frac{72}{r}

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