How to Calculate CFM for Bathroom Fan

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Who doesn’t love a hot, steamy shower? Bathrooms tend to collect more moisture than other rooms in your home, and too much moisture is a recipe for mold and mildew. Fortunately, as long as you have an exhaust fan that can refresh the air, you won’t have any problems. The key is to figure out how strong of a fan your bathroom needs by calculating the CFM, or cubic feet per minute, required to keep the air properly circulating. To make it simple for you, we’ve answered a few of the most common questions that folks have about how to calculate the CFM for a bathroom fan.

Question 1 of 5:

How do you calculate CFM for a bathroom fan?

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    Use a tape measure to measure each dimension of your bathroom - the length of each wall and the height from floor to ceiling. For a bathroom with a rectangular floor plan, you can keep these numbers in your head for the CFM calculation. If you have an irregular floor plan, you may want to draw and label the plan on paper to help you calculate your bathroom's volume.
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    For a rectangular bathroom, this is done by multiplying the width and length. [1] For example, a 7 ft x 10 ft (2.1 m x 3 m) bathroom has a floor area of 70 square feet (6.5 square meters). If your bathroom has a more complex shape, break the floor plan into simpler shapes, calculate their areas, and then add those areas together to arrive at the total floor area. Don't ignore the air above the bathtub--it is part of the total volume.
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    Use this rule of thumb to make your calculations simpler. Choose a fan with a CFM that matches your area (or closely matches it). [2]
    • For instance, if your bathroom is 75 square feet (7.0 m 2 ), choose a bathroom fan with at least 75 CFM.
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    A higher ceiling means more total volume in your bathroom. Multiply your area by the height of your ceiling and divide that number by 60 (for 60 minutes in an hour). Round up to the next whole number and then multiply that number by 8 (the number of air exchanges in an hour) to get your CFM value. [3]
    • For instance, if your bathroom is 60 square feet (5.6 m 2 ) and your ceiling is 10 feet (3.0 m) tall, multiply them together to get 600. Divide 600 by 60 to get 10. Then multiply 10 by 8 to get 80 as the ideal CFM for your fan.
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    Larger bathrooms need an exhaust fan with a little more juice to properly ventilated the room. Find the total area and then add in the CFM requirements for each of these fixtures: [4]
    • 50 CFM for each toilet
    • 50 CFM for the shower
    • 50 CFM for a bathtub
    • 100 CFM for a jetted tub
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Question 2 of 5:

What does CFM mean?

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    CFM measures the volume of air in cubic feet for each minute that it moves. [5] It’s a useful way to measure how large and powerful you need your bathroom exhaust fan to be in order to prevent moisture from causing mold and mildew to form in the room. [6]
    • For instance, if you have an exhaust fan with a CFM of 50, then it moves 50 cubic feet of air each minute.
Question 3 of 5:

How much CFM do I need for my bathroom fan?

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    A fan with the right CFM will draw in enough air to help prevent moisture buildup. Choosing a fan with the right CFM is directly related to the total area of your bathroom and can also be affected by the height of your bathroom ceiling. [7]
    • You want a fan strong enough to pull all of the air out of your bathroom every 7½ minutes to keep moisture from building up.
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Question 4 of 5:

What CFM do I need for a small bathroom?

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    If you have a really small bathroom, you don’t need to find a specific CFM. Just go with a fan that has 50 CFM. It’ll get the job done without any necessary measurements. [8]
    • This is a great option for a small half-bath or a hallway bathroom.
Question 5 of 5:

Is more CFM better for a bathroom fan?

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    A fan with more CFM is going to be more powerful. But it can also be much noisier. Ideally, you want a fan with just the right amount of CFM to refresh the air in your bathroom that’s also as quiet as possible. [9]
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  • Question
    How many CFM do I need for my bathroom fan?
    Mitchell Newman
    Construction Professional
    Mitchell Newman is a Construction Professional based in Chicago, Illinois. He is the Principal at Habitar Design, an interior design firm that specializes in floor plans, kitchen and bath design, furniture layout, custom built-ins, custom and semi-custom cabinetry, staging, models, and facades. Habitar features a strong design-build capability, utilizing Mitchell’s sister company, Stratagem Construction. Mitchell is the General Contractor at Strategem, and has 20 years of experience in construction, interior design and real estate development. Mitchell’s two companies seamlessly combine interior design with construction, allowing him to design with synchony for the client. He received an M.D. from the University of Chicago Pritzker School of Medicine and a BS in Chemistry from the University of Illinois Urbana-Champaign.
    Mitchell Newman
    Construction Professional
    Expert Answer
    Typically, they have a minimum of 90. There might be some that are even 60. A lot of them are 90 or 100, and that's totally sufficient for a typical bathroom. I think if you have it, then it tends to be sufficient if you have 100.
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      Tips

      • If your calculations come up with an odd number, round up to the nearest CFM value when you’re choosing a fan so you’ll have enough to effectively refresh the air in your bathroom.
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      References

      1. ↑ https://www.todayshomeowner.com/bathroom-vent-fan-cfm-calculator/
      2. ↑ https://www.bobvila.com/articles/bathroom-fan-size/
      3. ↑ https://www.bobvila.com/articles/bathroom-fan-size/
      4. ↑ https://www.hvi.org/resources/publications/bathroom-exhaust-fans/
      5. ↑ https://www.vmacair.com/blog/whats-the-difference-between-psi-and-cfm/
      6. ↑ https://www.thisoldhouse.com/bathrooms/21223814/how-to-choose-bathroom-exhaust-fan
      7. ↑ https://www.thisoldhouse.com/bathrooms/21016422/how-to-upgrade-to-a-bigger-bath-fan
      8. ↑ https://www.hvi.org/resources/publications/bathroom-exhaust-fans/
      9. ↑ https://www.hvi.org/resources/publications/bathroom-exhaust-fans/

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