- 1). Measure the room and multiply its width and length together to calculate the square footage.
- 2). Allow 1 CFM (cubic feet per minute) of air for each square foot of space. So if you have a 200-square-foot room, that room requires 200 CFM of airflow.
- 3). Pull out your duct calculator and line up the required CFM with the friction loss value of 0.1 to find out what size duct you need. If you used the example above, note that the duct calculator shows you can use 8-inch duct to supply the space.
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