Home Improvement Expert Danny Lipford

Bathroom Vent Fan CFM Calculator


Bathroom exhaust vent fan with light.

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.

Room Width


Room Length


Room Height

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20 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.

    December 22nd, 2014 at 1:13 am


  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?

  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?

  10. scott keith Says:
    September 4th, 2015 at 9:57 am

    What about installation on a slanted ceiling? Will it damage the impeller bearings and will this installation provide and acceptable level of venting to avoid moisture removal?

  11. Melody Pastura Says:
    September 19th, 2015 at 1:30 pm

    our shower is 4′ x 5′. We want fan light in shower. How many cfm should I get? Our Home Depot sells 50, 70 or 100 cfm’s. Thx

  12. Green Builder Says:
    October 2nd, 2015 at 6:14 pm

    To Melody Pastura:

    Don’t know of any fans suitable for use IN the shower. There are lights made for wet locations that can be safely installed in the shower, but the fan should be outside the shower enclosure. Compute its cfm based on the area of the entire room.

  13. Ron Says:
    October 8th, 2015 at 10:09 am

    My vent hasn’t worked in years. It’s 35 years old. Will I have installation problems (new fan too large or too small) with the new fans of today!! Thank You for the information!!!

  14. ed drake Says:
    October 19th, 2015 at 2:03 pm

    I found a vent /light fan that says, Fan sound level (stones) is 3.0. what does that mean? Is it real quiet or real loud?

  15. Rob Says:
    October 19th, 2015 at 2:39 pm

    I have a 12 x 12 room with a hot tub. Is venting this 12 x 12 room sufficient to avoid mold/mildew? Should I also run a de-humidifier?

  16. Chaser Says:
    October 20th, 2015 at 6:39 am

    To Melody Pastura.

    If you look on line you can find fans and lights for installing over a shower.

  17. Chaser Says:
    October 20th, 2015 at 6:40 am

    To Ed Drake.

    3 Sones is very loud. You want to find a fan that is under 1 sone or less.

  18. Daryl Schubert Says:
    October 29th, 2015 at 8:06 pm

    I have an 8’x 8′ bathroom with a 80 cfm fan. When we take a hot shower, the ceiling and walls are wet. What can I do to fix this problem before my walls start peeling? It is a new fan.

  19. Chip Says:
    November 1st, 2015 at 1:09 pm

    Our bathroom (1.5M X 3M) has low ceilings (2.3 M). Will an 80 cfm fan pull in enough fresh air to alleviate the problem?

  20. Richard Says:
    November 4th, 2015 at 7:18 am

    I need a 62 cfm bathroom exhaust fan per calculated. I have 6 foot flex, standard diameter, going to a roof vent. What added fan cfm should I have for duct work? The roof vent does not have a back draft damper for back draft to bathroom. Should I have one and can I get a fan with back draft attatched to it or can I buy an inline back draft damper to add to the duck work?

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