The rule of 72 is a handy way to peg the years it will take for an investment to double. Take 72 and divide it by the compound interest rate. Earn 8% and you'll double your money every nine years. Close enough, anyway. In 1944,72 years ago, the U.S. gross domestic product was $225 billion (in current dollars). Today it's approximately $18 trillion. That represents an annual compound growth rate of 2.9%. If the U.S. economy had grown just one percentage point more per year, the rule of 72 says the size would be double what it is today. (Actually, a bit more, when you run the numbers: $37.1 trillion.)
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