Air velocity is a measurement of the displacement of air. When it comes to the fans in heating and air-conditioning systems, this is a measurement of the displacement of air through the attached ventilation system. If you're trying to calculate the air velocity of the fan in your HVAC system, you can do so by taking a few measurements of your system and performing some basic math.
Look on the technical specification sheet for your HVAC unit to find the air flow of the unit's fan. This measurement is expressed in "cubic feet per minute," which is abbreviated as "CFM."
Use a tape measure to find the width and length of the air duct the fan is attached to. These measurements need to be expressed in inches.
Divide the length and width measurements of your air duct by 12 to convert them to feet. For example, an air duct that is 48 inches in length is 4 feet in length. An air duct that is 12 inches in width is 1 foot in width. When you're done, multiply these units together to find the area in square feet of your air duct.
Divide the air flow (in CFM) of your air duct by the area (in square feet) of your air duct. The number you get is the velocity of the fan that is connected to the air duct, measured in feet per minute.