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

Hi 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,

Danny

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3 Comments on “How to Size a Bathroom Vent Fan”

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  • Ted Ramsay Says:
    June 26th, 2015 at 8:36 am

    Hi Tom,
    I am planning to install a bathroom fan and I would like to know of it is acceptable to vent directly through the soffit. This would be a very short run of pipe, but it would have to be directed downward. My other options are to run the pipe to the attic and out the ridge vent, or go through the roof. I plan to have a new roof installed, possibly metal, so I would prefer to locate the outlet elsewhere. Our house is brick veneer, so, going through the gable is impractical and would be unsightly on the front or the back of the house. Thank you for your advice.
    TR


  • Official Comment:


    Ben Erickson Says:
    December 30th, 2014 at 8:37 am

    Hi Tom,
    Either one will work, but most vent fans are installed in the ceiling of the bathroom, since warm air from a shower rises. The ones I put in in my house go into the attic, then vent out through the gable siding. The advantage is that you don’t have to cut a hole in your roof.



  • tom Says:
    December 29th, 2014 at 2:56 pm

    I want to install a bathroom fan in a bathroom that has never had a fan. Is venting threw the roof better then threw the wall


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