How to Size a Bathroom Vent Fan

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

Further Information

8 COMMENTS

    • 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.

  1. 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

  2. I overdo things being a builder, master plumber, HVAC contractor, electrical contractor and Asbestos Abatement contractor, that is long since retired.

    I’m building my last home out in the country, just something small and simple.

    I have decided to use 200 cfm exhaust fans for both full baths (5×10) and am wondering if it is too much?

    I don’t see why it would be too much but figured you may provide something to consider.
    CYa
    Stephen

  3. Hello –

    Our builder installed a Panasonic Whisper ventilation fan (80 cfm I think) in a small utility room. Unfortunately, the fan is located above where I need to install cabinets. How much space must I allow between the fan “grill” and the top of the cabinets?

    Thanks.

  4. Hi Tom
    I have a bathroom fan the tp stay up for a sec then blows back down is that cause the fan is not big enough or is it because I don’t have enough fresh from in air from underneath the door

  5. Hi Tom
    Also the bathroom is open to the laundry room do use the laundry room in my calculations for the size of fan I need the only door is to the bath room and just a opening to the laundry room

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