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

  1. 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

  2. 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).

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

  4. 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

  5. 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?

  6. 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?

  7. 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?

  8. 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

  9. 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.

  10. 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!!!

  11. 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?

  12. 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.

  13. 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?

  14. 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

  15. 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.

  16. 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?

  17. 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.

  18. 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.

  19. 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?

  20. 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?

  21. 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?

    • 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: https://todayshomeowner.com/radio/ask-questions/
      Take care!

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