Cubic feet per minute is calculated by multiplying air velocity by area. In the case of air conditioning units, the volumetric flow rate can be calculated by multiplying the air velocity by the area of the AC duct.
For instance, the area of a 10-inch diameter round duct is 0.55 square feet. This can be calculated using the equation for the area of a circle, A=pi*r^2, where "r" is the radius of the duct. If the air velocity is 2 feet per second, and CFM=A*V, where "A" is the duct area and "V" is air velocity, then CFM=2 fps X 0.55 square feet, or 1.1 CFM.