Rule of 72

A mathematical rule for determining the number of years it will take for an investment to double in value. The number of years is determined by dividing 72 by the annual rate of return. Thus, an investment expected to earn interest at a rate of 8 percent will double an investor's funds in 72/8, or nine years. Dividing 72 by the number of years in which an investor wishes to double his or her return will yield the necessary rate.