Investors commonly ask the question: How long would it take to double the amount !!!â€¦Here comes a Handy Rule of 72. It may give you a approx. time in which the money invested will be doubled. Say rate of interest is 9% & you want to know after how much time your money will double, just divide 72 by 9 to get the desired result; 72/9= 8 years. Find below a handy calculator to find the time in which your money will be doubled. You just need to enter the rate of interest. :