How to Calculate Airflow in Ductwork

Find the size of the ductwork. Measure two adjacent sides to find the ductwork's dimensions. For example, if the top of the duct is 12 inches across and the left side is 10 inches across, your duct size is 12-by-10-inch. You do not need to measure the right side and the bottom as they will match the other two sides.

Pull out your duct calculator. Using the example above, line the "12" under "rectangular duct dimensions" up with the "10" in the same color code. If using the Trane duct calculator, both of these would be color coded green.

Find the "0.1" in "friction per 100 feet of duct." This friction loss figure per 100 feet of duct is accurate for most home and small commercial heating and air conditioning systems. Follow this line over to the corresponding volume in cfm (cubic feet per minute). For a 12-by-10-inch duct, this value would be 700 cfm, which is the maximum airflow through that size ductwork. Once again, if using a Trane duct calculator, both of these would be color coded light blue.