Enter the duct flow rate, duct width and height and select the units for each input. The flow velocity and the flow rate in the duct will be displayed in the selected output units.

Air velocity is calculated by dividing the volumetric flow rate by the cross-sectional area of the air flow passage.

Velocity (LFM) = Volume (CFM) / area (ft²)
Velocity (m/s) = Volume (m³/s) / area (m²)

Air flow is usually provide by one or more fans. Although most fans are normally rated and compared at their free air delivery at zero back pressure, nearly all applications have flow resistance so the fans act against some back pressure. For axial fans the volumetric flow rate can be derated to about 60% to 80% of free air delivery to anticipate typical levels of backpressure.

Example The free air volume delivered by a fan is given as 80 CFM. The area of the air flow passage is 6 inches by 6 inches or 36 in² or 0.25 ft².

To find velocity: Velocity (LFM) = 80 CFM / 0.25 ft² = 320 LFM

Derated to 80% the Velocity would be 256 LFM. A derated flow rate can be entered in the velocity calculator by using a multiplier in the CFM input cell. For example enter 80*0.8 to derate 80 CFM to 80%.