How to Properly Size a Bathroom Vent Fan

How do you size a bathroom vent fan? -Sam

Bathroom exhaust vent fans are important, since they remove excess humidity from your bathroom to prevent mold and mildew. To determine the proper size vent fan you need for your bathroom:

  • Calculate the number of cubic feet in the bathroom by multiplying the room’s height x width x length in feet.
  • Divide the number of cubic feet in your bathroom by the number of minutes in an hour (60).
  • Multiply that number by the recommended number of air exchanges per hour (8).
  • The result is the minimum cubic feet per minute (CFM) rating for your vent fan, which will be listed on the fan.

Or use our online bathroom vent fan calculator.

Further Information


Sam Asks: How do you size a bathroom vent fan?

Danny Lipford: Your bathroom is a big source of moisture in your home, so it’s important to have the right size exhaust fan to remove all of that moisture. Now it’s very easy to determine exactly what size fan you need by grabbing your tape measure. Measure the length times the width times the height of your bathroom. Divide that number by 60, which is the number of minutes in an hour. Then multiply that number by eight, which is the suggested number of air exchanges that you should have in your bathroom each and every hour.

Now that number that you end up with will be your minimum CFM number or cubic feet per minute. Head on down to the home center, check out the exhaust fans, and find that number. Then buy an exhaust fan just a little bit bigger than that, and you’ll have the right fan for the job.


  1. When does a shower and size of it comes into the equation. I don’t think your calculator gives enough cams for this. Thanks

  2. Hello. I measured my bathrooms and the #’s don’t sound right??
    8.5×12.5×8 divided by 60×8=13.86?? This sounds like a really odd # for finding the correct fan for my bathroom. What am I doing incorrectly??
    Thanks for your tutorials on projects for the home and your morning program is great!!

    Tammi Brown

    • Hi, Tammi, here’s how it should break down:

      8.5 x 12.5 x 8 = 850
      850 divided by 60 = 14.17
      14.17 x 8 = 113.33

      So, you would need a bath vent fan rated at least 113 cubic feet per minute (CFM).

  3. How do you go about replacing an exhaust fan between the first and second floor (no access above) and get new exhaust to line up with old vent pipe/duct to outside ?

    • Hi, Jerry,
      Need more information about this topic? Connect one-on-one with a home improvement pro immediately through JustAnswer, a Today’s Homeowner partner:
      Good luck with that exhaust fan! 


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