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Using the Rule of 72, about how long does money take to double at 8 percent a year?

About 9 years, since 72 divided by 8 is 9.

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  1. 9 years ✔
  2. 12 years
  3. 18 years

Why: Divide 72 by the interest rate to estimate the years it takes money to double. At 8 percent that is 72 divided by 8, or 9 years. It is a quick estimate, not an exact answer.

Huh, didn't know that: The same trick works backward. If you want money to double in 6 years, divide 72 by 6 to see you need about 12 percent.

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References

Answer: 9 years. Divide 72 by the interest rate to estimate the years it takes money to double. At 8 percent that is 72 divided by 8, or 9 years. It is a quick estimate, not an exact answer.

  1. Rutgers NJAES: The Rule of 72 njaes.rutgers.edu/sshw/message/message.php?p=Finance&m=349
    it takes 9 years to double a sum of money with an 8% rate of return (72 divided by 8)
    Checked 2026-10-10.

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