The Rule of 72 is a handy mental-math shortcut that lets you quickly estimate how many years it will take for an investment to roughly double in value, without needing a calculator or detailed formula. You simply divide the number 72 by the annual rate of return you expect to earn, and the result is an approximate number of years to doubling.
For example, an investment growing at 12% a year would take roughly 72 ÷ 12 = 6 years to double, while one growing at a more modest 6% a year would take about 72 ÷ 6 = 12 years. While it's only an approximation and works best for moderate rates of return (roughly 6% to 15%), the Rule of 72 is a genuinely useful tool for quickly comparing the long-term impact of different expected returns, right in your head, without needing to run exact compound interest calculations.