How to Calculate the CFM for an Exhaust Fan
- 1). Calculate the volume of the room. Measure the length, width and height of the room in feet. Multiply these three numbers, which gives you the volume of the room in cubic feet. For instance, for a room that is 8 feet square and 8 feet tall, multiply 8 by 8 by 8 to get 512 cubic feet.
- 2). Determine how often you would like the exhaust fan to be able to exchange all of the air in the room. If you would like to have completely fresh air every five minutes, for example, then this is the exchange time.
- 3). Divide the number of cubic feet by the number of minutes you would like it to take to exchange all of the air in the room. This will give you the required CFM. In a 200-square-foot room, if you would like completely fresh air every five minutes, divide 200 by 5. This gives you a required 40 CFM for the exhaust fan.
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