For our application CFM is the measurement of air moving from point A to point B. When the air is pushed across a small opening we can measure how many cubic feet of air is crossing that opening per minute.
Example: a bathroom exhaust fan with a 4” round exhaust duct is our opening. If I said it exhausts 90 cfm of air, that means every 60 seconds 100 (12” High x 12” Wide x 12” deep) cubes of air are moved from the bathroom to outside. Or 1.5 one foot boxes of air every second.
The rule of thumb for a 90 square foot bathroom (square feet not cubic feet) is 90 cfm exhaust fan. If you have an eight foot ceiling in your 90 sq. ft. bathroom. You now have a 720 cubic feet of space.
720 cubic / by 90 cfm = 8
It will take 8 minutes to remove all of the air from your bathroom at one time.
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What does cfm stand for in regards to exhaust fans?
cfm = cubic feet per minute as stated by all the above. To make the term relevant we need to apply it to your use. Let’s say a bathroom is 8’L x 8’W x 8’H. 8x8x8 = 512 cubic feet. If you buy an exhaust fan that is only 50 cfm, it will take more than ten minutes to change the air in the bathroom. The primary purpose of a bathroom exhaust fan is moisture removal from the bath or shower, not odor removal. You want that fan to remove the steam and humidity before it can condense on the walls and fixtures. Figure out the volume of your room, then get a fan that can change that volume of air in five or six minutes.