Home Improvement Expert Danny Lipford

Bathroom Vent Fan CFM Calculator

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Installing a bathroom vent fan.

Bathroom vent fans serve an important function by removing humid air from your home to prevent mold and mildew from forming. Vent fans are rated by the number of cubic feet of air they move per minute (CFM), and it’s important to buy a large enough fan for the size of your bathroom.

Use our handy online calculator to see what size vent fan you need for your home. Simply enter the dimensions of your bathroom in the calculator below to determine the minimum size vent fan you need for the bathroom in your home.

When installing a vent fan, make sure there’s at least a 1/2″ gap under the bathroom door to allow fresh air to enter the room when the fan is running. Run the fan for 15 to 20 minutes after showering or bathing to expel all the excess moisture.

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9 Comments on “Bathroom Vent Fan CFM Calculator”

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  1. Leo Says:
    September 17th, 2014 at 7:34 pm

    Can I cause a problem with too large a CFM rating. Ex: If 68 CFM is what I need would one at 110 CFM be ok?

  2. Kirk Says:
    September 25th, 2014 at 8:35 pm

    Not a problem, fan will pull out excess stale air from the surrouding areas.

  3. Kirk Says:
    September 25th, 2014 at 8:41 pm

    Slightly open a nearby window to avoid negative pressure. Great for your home.

  4. MURRAY ENTERPRISE Says:
    December 22nd, 2014 at 1:13 am

    I HAVE A HOT TUB IN A ROOM THE ROOM SIZE IS 15 FEET WIDE 15 FEET LONG 9 FEET HIGH IN ATLANTA GA I NEED TO KNOW WHAT CFM FAN I NEED WHAT SIZE DUCT AND DO I NEED FRESH AIR IF SO WHAT SIZE PIPE THANKS

  5. Lander Says:
    March 5th, 2015 at 12:54 pm

    The calculation should also take into account (add to CFM requirement) the resistance of the duct (length in ft, x1.25 if flex duct), number of bends (# x 15 ft), and the roof/wall cap (+30 ft).

  6. Jennifer Says:
    June 9th, 2015 at 4:40 pm

    Our master bath opens to our bedroom (no door separating the rooms) should I take this into account when choosing a fan?

  7. Neeraj Bhai Says:
    June 16th, 2015 at 1:38 pm

    A scientifically presented article that prompts me to ask a somewhat unrelated question. Though the article is for bathroom ventilation my question is somewhat tangential. I have a small hall of 33 ft x 33 ft x 12 ft (plus a pyramidal roof above that). During the summer the inside observed temperature is 34 celsius. Outside temperature is 27 C. Is it possible to use exhaust fans to bring down the temperature to say 29 C?

    Assume only 1 door to the hall (9 ft by 8 ft). Will it be of help if we deploy a set of fans at the lower height to pump in the outside air and another set at higher height to take the hot air out? (AC is not an option for us)

    How to figure out the CFM required? How to ensure good air current through out the hall? Is there another way to bring the temperature down?
    Thanks

  8. dov berger Says:
    July 7th, 2015 at 11:25 pm

    is it possible that the vent fan i bought is actually bringing outside air in and not ventilating the air out? what do i do to correct?

  9. David A Turner Says:
    July 15th, 2015 at 5:44 pm

    Is there a simple less back & arm breaking way to clean an old Bathroom exhaust fans ‘squirrel cage’ ? Than just using a vacumn and a tooth brush for hours?

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