How to Calculate CFM for Bathroom Fan

Co-authored by Mitchell Newman

Last Updated: December 29, 2021 Approved

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?

Download Article

{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/a\/a6\/Calculate-CFM-for-Bathroom-Fan-Step-1.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-1.jpg","bigUrl":"\/images\/thumb\/a\/a6\/Calculate-CFM-for-Bathroom-Fan-Step-1.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-1.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}

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. ^{[1]
X
Research source}
{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/0\/00\/Calculate-CFM-for-Bathroom-Fan-Step-2-Version-3.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-2-Version-3.jpg","bigUrl":"\/images\/thumb\/0\/00\/Calculate-CFM-for-Bathroom-Fan-Step-2-Version-3.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-2-Version-3.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>\u00a9 2024 wikiHow, Inc. All rights reserved. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. This image is <b>not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. This image may not be used by other entities without the express written consent of wikiHow, Inc.<br>\n<\/p><p><br \/>\n<\/p><\/div>"}

For a rectangular bathroom, this is done by multiplying the width and length. ^{[2]
X
Research source} 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.

Advertisement
{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/a\/ab\/Calculate-CFM-for-Bathroom-Fan-Step-3-Version-2.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-3-Version-2.jpg","bigUrl":"\/images\/thumb\/a\/ab\/Calculate-CFM-for-Bathroom-Fan-Step-3-Version-2.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-3-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>\u00a9 2024 wikiHow, Inc. All rights reserved. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. This image is <b>not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. This image may not be used by other entities without the express written consent of wikiHow, Inc.<br>\n<\/p><p><br \/>\n<\/p><\/div>"}

3
Go with 1 CFM for every 1 square foot (0.093 m ² ) of floor space. Use this rule of thumb to make your calculations simpler. Choose a fan with a CFM that matches your area (or closely matches it). ^{[3]
X
Research source}
- For instance, if your bathroom is 75 square feet (7.0 m ² ), choose a bathroom fan with at least 75 CFM.
{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/9\/9e\/Calculate-CFM-for-Bathroom-Fan-Step-4-Version-2.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-4-Version-2.jpg","bigUrl":"\/images\/thumb\/9\/9e\/Calculate-CFM-for-Bathroom-Fan-Step-4-Version-2.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-4-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>\u00a9 2024 wikiHow, Inc. All rights reserved. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. This image is <b>not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. This image may not be used by other entities without the express written consent of wikiHow, Inc.<br>\n<\/p><p><br \/>\n<\/p><\/div>"}

4
Add additional CFM if your ceiling is higher than 8 feet (2.4 m). 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. ^{[4]
X
Research source}
- 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.
{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/5\/5b\/Calculate-CFM-for-Bathroom-Fan-Step-5-Version-2.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-5-Version-2.jpg","bigUrl":"\/images\/thumb\/5\/5b\/Calculate-CFM-for-Bathroom-Fan-Step-5-Version-2.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-5-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>\u00a9 2024 wikiHow, Inc. All rights reserved. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. This image is <b>not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. This image may not be used by other entities without the express written consent of wikiHow, Inc.<br>\n<\/p><p><br \/>\n<\/p><\/div>"}

5
Include fixtures for bathrooms larger than 100 square feet (9.3 m ² ). 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: ^{[5]
X
Research source}
- 50 CFM for each toilet
- 50 CFM for the shower
- 50 CFM for a bathtub
- 100 CFM for a jetted tub

Question 2 of 5:

What does CFM mean?

Download Article

{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/8\/8e\/Calculate-CFM-for-Bathroom-Fan-Step-1-Version-2.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-1-Version-2.jpg","bigUrl":"\/images\/thumb\/8\/8e\/Calculate-CFM-for-Bathroom-Fan-Step-1-Version-2.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-1-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>\u00a9 2024 wikiHow, Inc. All rights reserved. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. This image is <b>not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. This image may not be used by other entities without the express written consent of wikiHow, Inc.<br>\n<\/p><p><br \/>\n<\/p><\/div>"}
It stands for cubic feet per minute. CFM measures the volume of air in cubic feet for each minute that it moves. ^{[6]
X
Research source} 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. ^{[7]
X
Research source}
- 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?

Download Article

{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/2\/26\/Calculate-CFM-for-Bathroom-Fan-Step-2-Version-2.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-2-Version-2.jpg","bigUrl":"\/images\/thumb\/2\/26\/Calculate-CFM-for-Bathroom-Fan-Step-2-Version-2.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-2-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>\u00a9 2024 wikiHow, Inc. All rights reserved. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. This image is <b>not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. This image may not be used by other entities without the express written consent of wikiHow, Inc.<br>\n<\/p><p><br \/>\n<\/p><\/div>"}
You need enough to refresh the air at least 8 times every hour. 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. ^{[8]
X
Research source}
- 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.

Question 4 of 5:

What CFM do I need for a small bathroom?

Download Article

{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/c\/cb\/Calculate-CFM-for-Bathroom-Fan-Step-6.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-6.jpg","bigUrl":"\/images\/thumb\/c\/cb\/Calculate-CFM-for-Bathroom-Fan-Step-6.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-6.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>\u00a9 2024 wikiHow, Inc. All rights reserved. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. This image is <b>not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. This image may not be used by other entities without the express written consent of wikiHow, Inc.<br>\n<\/p><p><br \/>\n<\/p><\/div>"}
Choose a fan with 50 CFM for a bathroom less than 50 square feet (4.6 m ² ). 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. ^{[9]
X
Research source}
- 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?

Download Article

{"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/9\/91\/Calculate-CFM-for-Bathroom-Fan-Step-7.jpg\/v4-460px-Calculate-CFM-for-Bathroom-Fan-Step-7.jpg","bigUrl":"\/images\/thumb\/9\/91\/Calculate-CFM-for-Bathroom-Fan-Step-7.jpg\/aid1608085-v4-728px-Calculate-CFM-for-Bathroom-Fan-Step-7.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>\u00a9 2024 wikiHow, Inc. All rights reserved. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. This image is <b>not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. This image may not be used by other entities without the express written consent of wikiHow, Inc.<br>\n<\/p><p><br \/>\n<\/p><\/div>"}

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. ^{[10]
X
Research source}

Expert Q&A

Search

Add New Question

Question

How many CFM do I need for my bathroom fan?

Mitchell Newman
Construction Professional
Mitchell Newman is the Principal at Habitar Design and its sister company Stratagem Construction in Chicago, Illinois. He has 20 years of experience in construction, interior design and real estate development.

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.

Thanks! We're glad this was helpful.
Thank you for your feedback.
If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow
Yes No

Not Helpful 7 Helpful 3

Ask a Question

200 characters left

Include your email address to get a message when this question is answered.

Submit

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.

Thanks
Helpful 0 Not Helpful 0

Submit a Tip

All tip submissions are carefully reviewed before being published

Submit

Thanks for submitting a tip for review!

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

Log in

How to Calculate CFM for Bathroom Fan

Steps

How do you calculate CFM for a bathroom fan?

What does CFM mean?

How much CFM do I need for my bathroom fan?

What CFM do I need for a small bathroom?

Is more CFM better for a bathroom fan?

Expert Q&A

Tips

You Might Also Like

References

About This Article

Reader Success Stories

Did this article help you?

Quizzes

You Might Also Like

Featured Articles

Trending Articles

Featured Articles

Featured Articles

Watch Articles

Trending Articles

Quizzes

Explore Categories