The Rule of 72 is a simplified method for calculating approximately how long it will take for an investment to double. In order for the rule to work, you have to know the rate of return on the investment, the rate of return has to stay constant, and you can’t add or take away any money from the investment.

The rule is simple: just divide 72 by whatever your rate of return is and you’ll get the number of years it’ll take the investment to double. For example, if your rate of return is 3%, you divide 72 by 3 and get 24. So your investment will double in about 24 years, assuming the rate of return doesn’t change and you leave the investment alone until then.