How To Size A Bathroom Exhaust Fan (CFM Sizing Guide)

Do you want to learn how to properly size a bathroom fan?

Rated in CFMs, the size of your bathroom fan will determine whether it is underpowered, overpowered, or just right.

In this guide, I will go over...

  • How to size a bath fan for small bathrooms
  • Calculating CFM for large bathrooms with multiple fixtures
  • Why a too high CFM can cause negative air pressure problems

Let's get started with this guide!

how to size a bathroom exhaust fan

Bathroom Exhaust Fan Size 'CFM Rating'

Bathroom exhaust fan are rated in CFMs which stands for cubic feet per minute. Basically, CFM is a rating that will tell you how much the bath fan can exhaust in one minute.  Most bathroom fan sizes range in the 50-cfm to 180-cfm range.

I have never seen a bathroom fan that is smaller than 50-cfm, and I have rarely seen one in a residential home that is greater than around 200 cubic feet per minute.

Risks Of The Wrong CFM

The downsides of installing an under-powered bathroom fan is that it won't effectively remove the moisture and smells from the room.

One of the big problems with installing an over-powered bathroom fan is that it may cause negative air pressure problems in the home such as hazardous backdrafting of gas appliances.

For backdrafting to occur, you would probably need a very powerful bathroom fan (or multiple). It is also more likely to occur with an over-powered range hood or attic fan (not a bath fan) but it is something to keep in mind.

Read Also: How To Vent A Bathroom Fan Into An Attic?

What Bathroom Exhaust Fan Size Do I Need?

An important guideline for bathroom exhaust fan sizing is from the Home Ventilating Institute, an industry non-profit.

Bathrooms Less Than 100 Square Feet

HVI recommends that homeowners have a minimum of 1-CFM for every square foot for bathrooms in the 50-sf to 100-sf floor size range. So if your bathroom is 60 square feet, HVI recommends a minimum of 60-CFM. And quite frankly, for small bathrooms, you will have a hard time finding a bathroom fan that is less than 50-CFM anyways.

Bathrooms Larger Than 100 Square Feet

For larger bathrooms of 100-sf or more, HVI recommends that you add up the CFM based on the number of plumbing fixtures—rather than calculating based on floor size.

Read Also: What Are The Best Bathroom Fans With Heaters?

Bathroom Floor Size In Square Feet

Minimum CFM

50-sf Or Less

50-CFM

50-sf to 100-sf

1-CFM Per Square Foot

100-sf Or Higher 

Toilet = 50-CFM

Shower = 50-CFM

Bathtub = 50-CFM

Jacuzzi Tub = 100-CFM

***Add together each fixture

under-sized bathroom exhaust fans

Should I Consider a Secondary Bathroom Exhaust Fan?

If your bathroom is large enough, it may be important to invest in a second bathroom exhaust fan.

For example, if a bathroom has a separate toilet area (with a door), it’s never a bad idea to set up an additional fan. This ensures that the enclosed space is properly taken care of. Otherwise, air in that part of the bathroom will go untouched leading to unwanted odors and mold.

When measuring for a secondary space, it’s best to use the same calculation as before.

Standard 50-CFM Bath Fan

However, most secondary spaces such as enclosed toilet areas require nothing more than a simple 50 CFM bathroom exhaust fan.

Remember, the primary exhaust fan is already regulating the rest of the bathroom.

If you don’t want to install a secondary fan, it’s highly recommended to leave the toilet door at least partially open throughout the day. This allows the primary bathroom exhaust fan an opportunity to do its job. While it won’t be as effective as a secondary fan, there’s still a chance to see satisfactory changes in air quality.

Read Also: How To Replace A Bathroom Fan Without Attic Access?

Final Thoughts On Bathroom Fan Size

Proper bathroom fan sizing is an important decision and comes with several requirements. This is why it’s smart to take the measurements as early as possible before spending hard-earned money on a brand-new fan.

By using the formulas mentioned, the fan will work like a charm.

how to size a bathroom exhaust fan
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2 thoughts on “How To Size A Bathroom Exhaust Fan (CFM Sizing Guide)”

  1. Hi I have had a new roof put on my house in the last 5yrs. But know Iam replacing the Gutters but should the people be call out about the air vent leaking in the bathroom when it rains . Should I call the one that put the roof on.

    Reply
    • Hi Anita,

      By “air vent” I assume you are talking about the bathroom exhaust fan? Sometimes bathroom fan vents go all the way up to the roof.

      You could ask the gutter company to fix the leaking bath vent cover if they are licensed to do roofing work — otherwise I would call the roofing company. If you received a workmanship guarantee from the roofing company, having a different company fix anything on the roof may void the warranty. If the roof warranty is just for materials, then you are out of luck.

      Since the gutter company is going to be up there with tall ladders anyways, I would still at least ask them to inspect the bathroom vent cover. There is probably some crack in the flashing or a defective flap.

      Good luck!

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Arie Van Tuijl

Arie Van Tuijl

I am a licensed home inspector in two U.S. states and the founder of Home Inspector Secrets. After performing hundreds of inspections, and seeing thousands of house defects, I realized people would love to have an online resource dedicated to home maintenance. I write detailed home guides and product reviews sprinkled with inspection tips. You can read my bio here.

About Home Inspector Secrets

Home Inspector Secrets is an online resource for owners, buyers, and sellers to understand all aspects of home maintenance. We have detailed home guides, product reviews, inspection advice, and much more.

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Looking to learn more about home maintenance? Check out our other informative home product reviews and guides!