This is Pat Haggerty with Larchmont Wealth, defining finance, The Rule of 72, is one of the most useful equations you can use on a regular basis and this is how it works. You take 72 divided by the annual assumed rate of return. For example, if you are assuming a 10% annual rate of return, you take 72 divided by 10, and it means that in 7.2 years your investment will double.