How Accurate Is the Rule of 72?

To estimate how many years it will take for an investment to double in value, you can divide 72 by the annual interest rate you might expect to receive (assuming tax-free annual compounding).

          72
____________ =   Number of years money will take to double in value
  Interest Rate

Here is a comparison of Rule-of-72 estimates to results obtained from a financial calculator, which proves that the Rule of 72 is close enough for most purposes:

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