The speed of air through a duct is measured in:

The measurement of air speed in a duct is typically expressed in feet per minute. This unit is practical for assessing airflow rates in HVAC systems, where understanding the velocity of the air helps to determine system efficiency, comfort levels in occupied spaces, and overall performance. Feet per minute allows for a direct comparison with air volume measurements and is commonly used in engineering and design practices related to heating, ventilation, and air conditioning.

Other units like feet per hour or inches per minute may be relevant in specific contexts but do not provide the same level of granularity for air velocity in typical duct applications. Feet per second is less common for ductwork measurements, as it is more often used in situations where very rapid airflow is being discussed. The use of feet per minute strikes a balance that facilitates practical calculations and system design in the context of typical building ventilation systems.