Cubic feet per minute (CFM), is the specification used to help determine the appropriate fan to use in a particular situation by measuring the amount of air the fan can move in a certain time frame. CFM is how many cubic feet of air can be moved in one minute elapsed time. Air flow can be computed in two different ways; the first is using an air flow sensor and the second is using the air flow equation shown in the slide; (Q2/Q1) = (RPM1/RPM2), where Q1 is the air flow rate at RPM1 and Q2 is the air flow rate at RPM2.