How to Size a Bathroom Exhaust Fan

How to Size a Bathroom Exhaust Fan

If you want to learn how to size a bathroom exhaust fan, then you have come to the right place! Bathroom exhaust fans are a home product that generally goes unnoticed, especially since it could be so routine to turn them on for when you shower. But what a lot of people don’t know is that these bathroom fans are required by the NEC, the National Electrical Code, for bathrooms that don’t have windows for ventilation and are even sometimes required in bathrooms that do have a window, depending on the jurisdiction. Bathroom exhaust fans are required because bathroom exhaust fans work to remove hot, moist air and any odors that are in your bathroom. 

Whether you’re renovating your bathroom or simply replacing an old bathroom fan, installing a fan that is both weak and undersized is not worth the effort. Not only is it important to choose a fan that meets the minimum requirements of the NEC, but it’s also great to keep in mind the sizing so that it performs to the standards in which the best bathroom fans can. 

How to Size a Bathroom Exhaust Fan: Standard Fan Sizing 

How to Size a Bathroom Exhaust Fan: Standard Fan Sizing

What we mean by sizing refers to the amount of power that the fan has as opposed to the fan’s literal physical dimensions. The basis of creation for bathroom fans is the amount of air that they can move, which is measured in cubic feet per minute (CFM). In general, standard fan sizing applies to bathrooms that are 100 square feet large or less, but the rule of thumb is that you need at least 1 CFM per square foot of the bathroom area. For example, if your bathroom is 65 square feet large, then it’s preferable for your bathroom fan to have a rating of at least 65 CFM. 

The minimum allowed fan size is 50 CFM so even if your bathroom is only 40 square feet, you will still have to purchase a bathroom fan that is 50 CFM. But in general, it’s a good idea to oversize your bathroom fan slightly so that you’re sure that it performs well enough for the bathroom space that you have. Especially if your bathroom has a separate room or a jetted tub, then it may also be preferable if you had more than one fan as well. 

Sizing For Large Bathrooms

But what about if your bathroom exceeds the size of the standard 100 square feet? If this is the case, the sizing of your bathroom fan will rely heavily on the number of fixtures in your bathroom. This refers to the showers, tubs, and toilets that are in the room. Follow this general guide with each fixture’s designated CFM ratings: 

  • Toilet: 50 CFM
  • Shower: 50 CFM
  • Bathtub: 50 CFM
  • Jetted tub: 100 CFM

For example, if you only had a toilet and a shower, then you will require a bathroom fan that has a rating of 100 CFM, whereas if your bathroom has these two fixtures in addition to a jetted tub, then you will need a fan with a 200 CFM rating. 

As we mentioned before, if your bathroom has another enclosed area that’s in the form of a separate room, then you may have to invest in a second bathroom fan. If the space is small, then a 50 CFM fan should be enough to free the air from odors or humidity efficiently. If you believe that the fan needs a fan rating of more than 50 CFM, then do the appropriate calculations that we’ve mentioned before. 

Why Do I Need a Bathroom Fan?

As mentioned before, bathroom exhaust fans are designed to reduce the amount of humidity and odor present within your bathroom. One of the greatest benefits of having a bathroom fan is preventing mold buildup on your bathroom walls and ceiling. If you’ve noticed that the paint has been peeling off of your bathroom walls, then that means that your current bathroom fan may not be working efficiently enough to rid your bathroom of the moisture that is present. If you continue to allow mold to build up, then that could cause serious structural integrity issues which will cost you a lot to repair in the future. 

How To Test The Quality of Your Bathroom Fan

To test whether you’re ready for a new bathroom fan, you’ll have to perform the toilet paper test. To do this, you will take a piece of toilet paper and hold it up to the fan’s vent while it is on. If the fan holds the piece of toilet paper up on its own by suctioning it, then that means that your fan is working properly and is good to remove any humidity from your bathroom. If your fan does not hold the toilet paper up, that means that it’s not very good at clearing humidity. 


A white minimalist bathroom.

Overlooking your bathroom fan is easy to do, but it’s also relatively easy to replace if you know the dimensions of your bathroom. Sizing the bathroom exhaust fan to meet requirements and so that it performs adequately enough is important to prevent humidity buildup, which then prevents mold from growing on your walls and ceiling. Now that you know how to size a bathroom exhaust fan, take some time and try the toilet paper test! 

Written by Julia Batista

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