Cubic feet per minute, or CFM, is calculated by using a combination of air velocity and volume measurements. CFM is usually used to measure how much air flows through a given heating or cooling system in a set amount of time.
Air velocity is measured in linear feet per minute, or LFM, and can be measured as air passes through an air duct. By multiplying this number by the volume of the duct in cubic feet, a CFM measurement is obtained. Because air ducts may change size from point to point, these measurements should be taken over a few areas and then averaged to get an accurate CFM measurement.