The compound interest rule is when you have money invested, and they pay interest to you because they use your money,. Therefore every year you gain money from interest. Basically you gain more money by the interest you get paid.
To estimate the number of periods required to double an original investment, divide the most convenient rule-quantity by the expected growth rate, expressed as a percentage.
For example, if you were to invest $100 with compounding interest at a rate of 9% per annum, the rule of 72 gives 72/9 = 8 years required for the investment to be worth $200, an exact calculation gives 8.0432 years.
