A shortcut for estimating how long it will take for an investment to double in value at a given annual rate of return. Simply divide 72 by the annual return. For example, an investment earning 6% per year would double in approximately 12 years (72 ÷ 6 = 12). The rule can also be reversed to estimate the rate of return needed to double money within a certain number of years, making it a quick tool for comparing the long-term impact of different rates of return. See Compound Interest