A shortcut to estimate the number of years required to double your money at a given annual rate of return. Divide the rate, expressed as a percentage, into 72. Years required to double an investment = 72 ÷ compound annual interest rate. For example, if you start with $1,000 and want to know how long it will take to double to $2,000 at an interest rate of 20%, then the calculation is: 72 ÷ 20 = 3.6 years.
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