The rule is to divide 72 by the rate of growth (or the interest rate, for savings and investments): the result gives you the number of periods it will take for the initial investment to be doubled. For instance, for an interest rate of 9% a year, we divide 72 by 9 and get 8 years. The actual time it would take money to double at 9% is 8.043 years (see Figure 5), so this is reasonably accurate