Doubling time and the Rule of 72

To estimate the time (number of periods) required to double an original quantity, divide 72 by the expected growth rate, expressed as a percentage.

• For instance, 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
• If you can consistently make 12% per month trading the Forex market, you will need 72/12 = 6 months to double your initial investment.

Moreover, to estimate the time it takes to triple the original quantity, use 114 instead of 72. 
To estimate the time it takes to quadruple the original quantity, use 144.