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What is the Rule of 72?

Rule of 72

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Rule of 72

The Rule of 72 is a quick, simple formula used to estimate the number of years required to double the value of an investment at a fixed annual rate of return or compound interest. The rule provides a way to determine how long an investment will take to double, given a consistent interest rate or return.

The formula is:
Number of Years = 72 / Annual Rate of Return (as a percentage)

The Rule of 72 is especially useful because of its simplicity, allowing for quick mental calculations without the need for a calculator or more complicated formulas.

Example of the Rule of 72

Imagine that Emily has just started her career and decides to invest in a mutual fund that historically offers an average annual return of 9%.

Emily wants to understand approximately how long it will take for her initial investment to double in value using this annual return rate.

Using the Rule of 72:

To estimate the number of years required to double her money at a 9% annual return:

Years to Double = 72 / Annual Rate of Return

Years to Double = 72 / 9 = 8

According to the Rule of 72, Emily’s investment will roughly double in 8 years with an average annual return of 9%.


In this scenario, the Rule of 72 provides Emily with a straightforward way to estimate her investment’s growth over time. While the rule offers a close approximation, it’s essential for Emily to understand that actual investment performance can vary due to various factors. However, the Rule of 72 gives her a general idea, allowing her to make informed financial decisions based on her long-term goals.

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