What Size Bathroom Vent Fan Do I Need

We've all been there.

Y'all walk into the bathroom, and the fan is so weak it barely moves whatsoever air at all.

On the flip side, an overly powerful bathroom fan can be needlessly loud and expensive.

Well, if yous plan on buying a new bathroom fan, it is important to size it correctly.

Bathroom Exhaust Fan Sizing Chart

The 'size' of a bathroom fan doesn't actually refer to the concrete dimensions but rather to information technology's CFM rating.

CFM stands for cubic feet per minute, and it basically tells you how much air the bath fan can move in 1 minute.

Most bath fans have CFM ratings in the range of 50-cfm to 130-cfm but can go equally high as 200-cfm or greater.

There are a few dissimilar ways to calculate the CFM for your bathroom, just all methods try to aim for at least 8 air changes per hr. This means that the bathroom'south air book should be totally replaced at to the lowest degree viii times in hour.

Information technology's also really important to brand certain that the bath door has a satisfactory gap on the lesser. If the bottom door gap isn't at least 5/8-inch, information technology volition exist difficult for the bath fan to properly practise information technology's chore because there isn't enough makeup air.

In this guide on bath fan sizing, I will go over:

the four primary means to calculate CFM (andexhaust fan size nautical chart)
a handy CFM estimator for the most complex method
things that tin can affect CFM (similar the duct size, motor quality)
loftier CFM bath fans with low dissonance levels
how a small vent pipe size tin reduce CFM output

Four Means To Size An Exhaust Fan (CFM)

At that place are 4 primary ways to size a bath fan. These CFM calculations are based on the floor area, ceiling height, and boosted enclosed spaces like a toilet or shower.

In the exhaust fan size chart below, you will see a quick breakdown of each method, and you lot can go along reading for full details on each calculation:

Bath Exhaust Fan Size Nautical chart

CFM Calculation	Minimum CFM
Regular Sized Bathrooms (100-sf or less)	1-CFM Per Square Pes (minimum 50-cfm)
Bath With Alpine Ceilings	1. Multiply flooring area by the ceiling height two. Split By 60 (minutes) three. Multiply by 8 (# of air changes)
Large Bathrooms (100-sf Or Higher)	Add together each fixture Toilet = 50-CFM Shower = l-CFM Bathtub = 50-CFM Jacuzzi Tub = 100-CFM
Enclosed Areas (h2o closet or shower)	50-cfm (second exhaust fan)

#1. Standard Bathroom Fan Size (100-sf Or Less)

The start and most common way to size a bathroom fan is to use ane-cfm per square pes of floor area. To calculate the floor area, simply accept a tape measure out and multiply the length and width of the bathroom including the shower/tub areas.

This standard bathroom fan size adding of 1-cfm per foursquare pes is for regular sized bathrooms that are 100-SF or less. For case, if you have a bath that is 80-sf, then you desire a bathroom fan that is at least 80-cfm.

If you take a very tiny bathroom that is less than 50-sf (such equally 40-sf), you want to keep a minimum of 50-CFM. And y'all tin can always go slightly more than than the recommended minimum of i-cfm per one-sf.

A somewhat higher CFM (like x-20% college) won't hurt anything, but an underpowered bathroom may not adequately remove the moisture needed to protect your bath.

If you are worried nigh the noise, y'all can buy a bath fan with a low sone rating such as 0.3-sones which is well-nigh whisper quiet. I recently wrote a product review on the best ultra quiet bathroom fans that have very low sone ratings.

#2. Bathrooms With High Ceilings (More than 8-anxiety)

In add-on to the standard CFM calculation, you lot will need to calculate CFM differently if yous have a standard sized bathroom (100-sf or less) with tall ceilings.

The standard ceiling height for bathrooms is 8-anxiety, so if you measure your ceiling and information technology is ten-feet or 12-feet, y'all will need to factor in the increased vertical space. Y'all tin can use my calculator beneath to find out the CFM:

And here is a breakup and example of how the sizing calculator works:

First Stride. Multiply floor SF past the ceiling height (Floor SF × Ceiling Height)
2nd Step. Divide By 60 (minutes)
Third Step. Multiply by eight (# of air changes)

As an example: if you take a bathroom with ten-ft ceilings and the floor area is 120-sf.

Get-go Pace. 120-sf × 10-ft = one,200
2d Step. one,200 / 60 = twenty
Third Stride. xx × 8 = 160 CFM

Yous would need a bath fan that is at least 160-cfm or two bathroom fans that equal 160-cfm. You tin too become a bathroom fan that has a slightly higher CFM (by 10-20%) than the minimum without information technology being a problem. I don't recommend ever going below the recommended exhaust fan CFM however.

#3. Large Bathrooms (More Than 100 Square Anxiety)

For larger bathrooms of 100-sf or more, we recommend that yous add together up the CFM based on the number of fitting rather than computing based on flooring size.

For each toilet, shower, or bathtub, simply assign 50-cfm to each fixture. If you have a jetted tub, assign 100-cfm.

Toilet = 50-cfm
Shower = l-cfm
Bathtub = 50-cfm
Jacuzzi Tub = 100-cfm

So if yous have a bathroom with one toilet, one shower, and one jacuzzi tub—the minimum CFM required would exist 200-cfm. You lot can either buy i bathroom fan rated at 200-cfm or yous can buy 2-3 fans that add upwards to 200-cfm.

#four. Enclosed Areas

If you have a bathroom that has an enclosed water cupboard or shower, you lot will need to buy a second bathroom fan just for that area.Since the toilet area or shower has a door, it volition forbid the bathroom fan from ventilating that infinite adequately.

If you take a completely enclosed shower area, the bath fan will also need to exist GFCI continued. Pretty much all water closets that I inspect accept a bathroom fan with a light also. Since the enclosed area is usually such a small expanse, going with a small 50-cfm bath fan is standard.

And if y'all admittedly don't want to install a secondary fan, information technology's recommended to leave the door at least partially open throughout the twenty-four hours. This allows the primary bathroom frazzle fan an opportunity to do its job.

Often Asked Questions

Here are a few oft asked questions that are related to sizing a bathroom fan:

Can A Bath Fan Be Too Strong?

The brusk respond is yes. If the bath fan is significantly stronger than the recommended CFM, there is a adventure it will cause backdrafting in the habitation. Backdrafting occurs when there are gas appliances in the habitation, and the powerful fan creates negative air force per unit area.

The negative air pressure can pull back in baneful gases back into the home from the gas appliances (instead of going outside) such as hazardous carbon monoxide. However, if you only increase the CFM 10-25% from the standard CFM recommendations, information technology should non exist problem.

Tin The Duct Size Touch CFM?

The quality of your duct absolutely can affect the CFM output of your bathroom fan. New bathroom fans will tell yous the vent pipage size (bore) that needs to be used such as iii-inch, 4-inch, or half-dozen-inch. The best ducts for bathroom fans also tend to exist pre-insulated which helps reduce noise and prevents condensation.

If the bath fan is rated for 130-cfm with a 6-inch duct, simply you only have 3-inch installed, information technology volition decrease the CFM performance.In improver to the duct size, the number of bends, overall length, and metallic vs. foil blazon of duct can have an impact on CFM.

What If I Am Concerned About Noise?

Bathroom fans come with a noise rating chosen sones. Anything that is 1.0 sone or less is considered quiet. In fact, a bathroom fan with a 0.iii sone rating can be difficult to know that it'southward fifty-fifty on which is why they have LED indicator lights. A bathroom fan that is iii-4 sones is pretty loud. My guide on bathroom fan sones details how information technology works.

A higher CFM bathroom fan will likely have a higher sone rating, but not all. Higher quality bathroom fans tend to as well accept premium motors (variable speed and ECM) that tin can significantly reduce noise.

Are At that place Bathroom Fans With Departure Speeds Or CFMs?

A lot of the bathroom fans made by Panasonic have a speed selector characteristic which means you can cull between three dissimilar CFMs such as 50/80/110. There is a little switch on the housing that allows you to choose which CFM yous want.

At that place are also bathroom fans with variable speed motors that kickoff off on a slow speed when yous commencement turn on the fan, and if information technology detects loftier moisture, it will kick into a higher speed. These fans have humidity sensors so they can automatically turn on or off as well. You can read my review on the all-time bathroom fans with humidity sensors here.