When would you need to use the rule of 72?

The "Rule of 72" is a method used to determine how long an investment will take to double, given a fixed annual rate of interest. It is a shortcut to estimate the number of years required to double your money at a given annual rate of return. By dividing 72 by the annual rate of return, investors can get a rough estimate of how many years it will take for the initial investment to duplicate itself.