This formula is useful for financial estimates and understanding the nature of compound interest. Examples:

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* To double your money in 10 years, get an interest rate of 72/10 or 7.2%.

The rule of 72 apply to exponential growth and are therefore used for compound interest as opposed to simple interest calculations. They can also be used for decay to obtain a halving time. The choice of number is mostly a matter of preference;

69 is more accurate for continuous compounding, while

The rule of 72 comes from a mathematics approximation, to be more exact, it comes from Taylor expansion for log function. It has very logic and rigious mathematics proof.

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