The rule of 72 is a quick and handy method widely used in finance to estimate how many years it will take to double the principle at a given annual return rate. While we can accurately calculate the time required for the investment to double using calculators and computer programming, the rule of 72 provides an approximate value with some simple mental math.
If an investment promises a 9% annual rate of return, then by the rule of 72, it will take approximately 8 years ($\frac{72}{9}=8$) to have the principle doubled.