How to Calculate the CFM for an Exhaust Fan

Updated April 17, 2017

An exhaust fan helps pull impure air out of your home and brings fresh air in. Generally these fans are connected by a duct to the exterior of the home, which helps to ensure you have high-quality indoor air at all times. Exhaust fans are rated in units of cubic feet per minute, or CFM. To calculate the CFM needed for an exhaust fan in a room, you will need to take a few measurements and do some simple calculations.

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.

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.

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.


Check the manufacturer's packaging to make sure you purchase a fan with the correct rating to match the CFM that you calculate.


If you're installing the exhaust fan in a bathroom, account for shower and tub space as well.

Things You'll Need

  • Tape measure
  • Calculator
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About the Author

David Somerset has been a writer intermittently for 11 years. He attended New Mexico Tech and earned a Bachelor of Science in technical communication in 2007. From being published in the "Bucksworth Community News" to writing how-to articles for eHow, his experience is quite diverse.