A duct calculator measures air velocity by using a formula in which air flow is a variable expressed in cubic feet per minute, or CFM. Heating, ventilation and air conditioning professionals commonly use duct calculators.
Air velocity is the measurement of the speed of the air flow, while air flow is the measurement of the volume of air at a specific point. Determining the velocity of air as it moves through duct work or pipe depends on multiple variables including duct size measurements, duct area and air flow. While calculators express air flow in cubic feet per minute, they express air velocity in linear feet per minute, or LFM.