- 1). Calculate the cubic footage of your bathroom. Measure the length, width and height of the bathroom. If the length is 10 feet, the width is 8 feet, and the height is 8 feet, the cubic footage would be 640.
- 2). Calculate the cubic feet per hour requirement for your bathroom exhaust. A bathroom needs eight air changes per hour. So if using the example from Step 1, multiply the eight air changes per hour by the cubic footage to get 5,120 cubic feet per hour.
- 3). Convert cubic feet per hour to cubic feet per minute (CFM), since fans are sized using CFM. Take the cubic feet per hour and divide by 60 minutes to get CFM. If using the example above, take 5,120 cubic feet per hour and divide by 60 to get 85 CFM. Look for an exhaust fan rated as close as possible to 85 CFM.
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