You would use the rule of 72 when you are calculating the value of an investment.
The rule of 72 is a way to simply calculate how long it will take for an investment to double in value. The dividing 72 by the annual rate of return you can roughly calculate how long it will take for an investment to double in value. For example, if you are early 8% annual interest on an investment it will take approximately 9 years for it to double in value (72/8=9).
