The Bogleheads' Guide to Investing

The Rule of 72 is very simple: To determine how many years it will take an investment to double in value, simply divide 72 by the annual rate of return. For example, an investment that returns 8 percent doubles every 9 years (72/ 8 = 9). Similarly, an investment that returns 9 percent doubles every 8 years and one that returns 12 percent doubles every 6 years. On the surface that may not seem like such a big deal, until you realize that every time the money doubles, it becomes 4, then 8, then 16, and then 32 times your original investment.