A mathematical rule used as a short-cut to calculate the approximate number of years it would take for an investment, assuming it yields a certain rate of annual return, to double in size.
The number of years can be calculated by dividing 72 by the annual rate of return on the investment. For example, if an investment earns a return of 6% annually, it would take a total of 12 years for it to double. While the rule might require minor changes for higher rates of return, it can still serve as a useful rule of thumb.
