There's a simple little math trick that will tell you how long it will take to double the money you've invested. It's known as the "Rule of 72" and here's how it works: simply divide 72 by the interest rate you earn on your investment and the answer will tell you how many years it will take to double the value! For example: $100 invested that earns 6% a year will be worth twice as much in 12 years. 72 / 6 = 12