How to Size a Bathroom Vent Fan
By: Danny Lipford
What size exhaust fan should we use in our small (5’ x 7’) bathroom? –Tam
Bathroom vent fans are rated by the cubic feet per minute (CFM) of air they move and should be sized to replace the air in the room at least eight times an hour. If your bathroom has an 8’ ceiling, the CFM rating for your fan should be as least as high as the number of square feet (5’ x 7’ = 35 sq. ft.) in the room. So in your case, a 50 CFM fan should work. A bathroom with a higher ceiling would require a larger fan.
To remove more moisture from the air when showering, use a bit larger fan (80-110 CFM). Also, be sure there is at least a 5/8” gap under the bathroom door to allow fresh air to enter the room when the door is closed.
If your bathroom has a higher ceiling, multiply the width x length x height of the room, divide by 60 (minutes in an hour) then multiply by 8 (number of air exchanges per hour). For example, a 10′ wide by 15′ long bathroom with a 10′ ceiling would need:
10x15x10 = 1500
1500 ÷ 60 = 25
25×8 = 200 CFM rated vent fan
Or use our online bathroom vent fan calculator.
Good luck with your project,