Bathroom Vent Fan 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.

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34 Comments on “Bathroom Vent Fan Calculator”

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  • Official Comment:

    Thomas Boni Says:
    May 12th, 2019 at 1:19 pm

    Hi, Ken,
    We’re stumped — maybe the humidity levels in the house are abnormally high to begin with?
    You can try blocking the gap under the door to see if that helps remove the moisture better.
    If that doesn’t work, we recommend submitting questions about unique situations like yours to the Today’s Homeowner Radio Show.
    Please use this form to contact Danny Lipford, America’s Home Expert, directly:
    Take care!

  • Ken Kitko Says:
    April 12th, 2019 at 10:44 am

    My sons bathroom is under 100 sq.ft. I put in a new fan rated at 130 cfm and he still has moister on the ceiling and walls. The ceiling height is 8 ft. The door is 5/8 “ above the threshold and they have even let the door open when showering and still get condensation. The exhaust line is not plugged and is only 6 ft from the fan. What else , if any , can we do?

  • Gabriel Says:
    January 22nd, 2019 at 12:01 am

    I have a 50 cfm can I upgrade with a 70 cfm would that be a problem

  • Official Comment:

    Thomas Boni Says:
    December 17th, 2018 at 3:21 pm

    Hi, John!
    What a great question! We recommend submitting unique questions like yours to the Today’s Homeowner Radio Show.
    Please use this form to contact Danny Lipford, America’s Home Expert, directly:
    Take care!

  • John Snider Says:
    December 2nd, 2018 at 8:57 am

    We get a log of condensation on the walls what taking hot showers.

    Can I cause a problem with too large a CFM rating. Ex: If 66 CFM is what I need would one at 110 CFM be ok?

  • mazood Says:
    December 1st, 2018 at 7:44 am

    can i connect 4 toilet for ventilation using 4 inch pvc pipes,and one exhaust fan fitted one the top common for 4 toilet

  • Donald Vail Says:
    August 25th, 2018 at 8:36 am

    The bathroom has a separate toilet room. Therefor there are two fans. 100 CFM fan with a 4″ exhaust port and back flow flap. The second fan is 150CFM with the ability to have a 4″ or 6″ vent pipe attached. If I can find a “Y” pipe with a 4″ and a 6″ entry, what size should the exit pipe be?

  • Kyle Says:
    December 24th, 2017 at 2:46 pm

    What size of ceiling fan bath room12×30 big corner spa tub two haed shower

  • Shelley Mason Says:
    November 3rd, 2017 at 11:11 am

    If our bathroom requires A 35 cfm can I still use A 100cfm exhaust fan. Would it b OK?

  • tim Says:
    May 27th, 2017 at 9:09 pm

    I don’t want any condensation on the wall so if I put a 290cfm in a 80 cfm rated room. Would that ruin the motor of fan or can this.

  • Exhaust Fans Says:
    March 29th, 2016 at 12:33 am

    following specifications:

    Fan Blade Diameter: 12”
    Fan Blade Material: GRP/Polypropylene
    Air Flow: 1500 Cubic Feet per Minute or more
    Supply Voltage: 220VAC/50Hz
    Motor Protection Class: IP65
    Operation: Continuous 24/7
    Required Quantity: 02 Nos.

  • Kevin Pletscher Says:
    March 14th, 2016 at 1:54 pm

    I installed a 5″ duct fan for bathroom exhaust The total length of duct to outside is less than 6 ft. The duct fan is rated 120 cfm free air. The fan won’t even push the damper open even with the spring on the exhaust damper removed. Can I add another fan in line to improve air flow or will one fan work against the other?

  • Paul N. Says:
    January 3rd, 2016 at 6:40 pm

    I have a 6×13 bath with a 7ft ceiling. The vent fan is a 110cfm. That should be more than adequate, but there is moisture in the medicine cabinet, as well as on the walls and ceiling after a shower.The duct used is the cheaper aluminum coated one and 4″ diameter. Should I replace duct work with the HD stuff like they use on dryers? Duct length to outside is about 6 ft. Thank You for your expertise.

  • Patrick B. Says:
    January 2nd, 2016 at 2:25 am

    I am building a 11x15x12 insulated metal building that we will be putting a hot tub into. We will be leaving the cover on the hot tub when not in use. My question is would I need to leave a exhaust fan on the whole time that we are in the hot tub or just when we open the cover and the humidity rises? What are my options? Would it also be necessary to install a dehumidifier in this room as well? Thank you for all your help

  • 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?

  • 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?

  • 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.

  • 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.

  • 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.

  • 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?

  • 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?

  • 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!!!

  • 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.

  • 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

  • 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?

  • 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?

  • 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?

  • 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?

  • 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?

  • 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).

    December 22nd, 2014 at 1:13 am


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

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

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

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

  • 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?

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Bathroom Vent Fan Calculator