The rule of 72 is a very useful rule to not only figure out how long it will take you to double your money but it also helps demonstrate how 1-2% increase in interest rates can speed up accumulation of wealth. To quickly calculate how long it will take to double your money you simply take 72 and divide it by the interest rate or yield. So if you invest money in the stock market and get the traditional 12% return that the historical averages dictates it will take 72/12 = 6 years to double your money. But if you have your money in a 3% CD it will take you 18 years to double.