We've all been there.
You walk into the bathroom, and the fan is so weak it barely moves any air at all.
On the flip side, an overly powerful bathroom fan can be needlessly loud and expensive.
Well, if you plan on buying a new bathroom fan, it is important to size it correctly.
The 'size' of a bathroom fan doesn't actually refer to the physical dimensions but rather to it's CFM rating.
CFM stands for cubic feet per minute, and it basically tells you how much air the bath fan can move in one minute.
Most bathroom fans have CFM ratings in the range of 50-cfm to 130-cfm but can go as high as 200-cfm or greater.
There are a few different ways to calculate the CFM for your bathroom, but all methods try to aim for at least 8 air changes per hour. This means that the bathroom's air volume should be totally replaced at least 8 times in 60 minutes.
It's also really important to make sure that the bathroom door has a satisfactory gap on the bottom. If the bottom door gap isn't at least 5/8-inch, it will be difficult for the bathroom fan to properly do it's job because there isn't enough makeup air.
In this guide on bath fan sizing, I will go over:
- the four main ways to calculate CFM (and exhaust fan size chart)
- things that can affect CFM (like the duct size, motor quality)
- high CFM bath fans with low noise levels
- how a small vent pipe size can reduce CFM output
Four Ways To Size An Exhaust Fan (CFM)
There are 4 main ways to size a bath fan. These CFM calculations are based on the floor area, ceiling height, and additional 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 can keep reading for full details on each calculation:
Bathroom Exhaust Fan Size Chart
Regular Sized Bathrooms (100-sf or less)
1-CFM Per Square Foot (minimum 50-cfm)
Bathroom With Tall Ceilings
1. Multiply floor area by the ceiling height
2. Divide By 60 (minutes)
3. Multiply by 8 (# of air changes)
Large Bathrooms (100-sf Or Higher)
Add together each fixture
Toilet = 50-CFM
Shower = 50-CFM
Bathtub = 50-CFM
Jacuzzi Tub = 100-CFM
Enclosed Areas (water closet or shower)
50-cfm (second exhaust fan)
#1. Standard Bathroom Fan Size (100-sf Or Less)
The first and most common way to size a bathroom fan is to use 1-cfm per square foot of floor area. To calculate the floor area, simply take a tape measure and multiply the length and width of the bathroom including the shower/tub areas.
This standard bathroom fan size calculation of 1-cfm per square foot is for regular sized bathrooms that are 100-SF or less. For example, if you have a bathroom that is 80-sf, then you want a bathroom fan that is at least 80-cfm.
If you have a very tiny bathroom that is less than 50-sf (such as 40-sf), you want to keep a minimum of 50-CFM. And you can always go slightly more than the recommended minimum of 1-cfm per 1-sf.
A somewhat higher CFM (like 10-20% higher) won't hurt anything, but an underpowered bath may not adequately remove the moisture needed to protect your bathroom.
If you are worried about the noise, you can buy a bath fan with a low sone rating such as 0.3-sones which is almost 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-feet)
In addition to the standard CFM calculation, you will need to calculate CFM differently if you have a standard sized bathroom (100-sf or less) with tall ceilings.
The standard ceiling height for bathrooms is 8-feet, so if you measure your ceiling and it is 10-feet or 12-feet, you will need to factor in the increased vertical space.
And here is a breakdown and example of how to calculate CFM:
- First Step. Multiply floor SF by the ceiling height (Floor SF × Ceiling Height)
- Second Step. Divide By 60 (minutes)
- Third Step. Multiply by 8 (# of air changes)
As an example: if you have a bathroom with 10-ft ceilings and the floor area is 120-sf.
- First Step. 120-sf × 10-ft = 1,200
- Second Step. 1,200 / 60 = 20
- Third Step. 20 × 8 = 160 CFM
You would need a bathroom fan that is at least 160-cfm or two bathroom fans that equal 160-cfm. You can also get a bath fan that has a slightly higher CFM (by 10-20%) than the minimum without it being a problem. I don't recommend ever going below the recommended exhaust fan CFM however.
#3. Large Bathrooms (More Than 100 Square Feet)
For larger bathrooms of 100-sf or more, we recommend that you add up the CFM based on the number of plumbing fixtures rather than calculating based on floor 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 = 50-cfm
- Bathtub = 50-cfm
- Jacuzzi Tub = 100-cfm
So if you have a bathroom with one toilet, one shower, and one jacuzzi tub—the minimum CFM required would be 200-cfm. You can either buy one bathroom fan rated at 200-cfm or you can buy 2-3 fans that add up to 200-cfm.
#4. Enclosed Areas
If you have a bathroom that has an enclosed water closet or shower, you will need to purchase a second bathroom fan just for that area. Since the toilet area or shower has a door, it will prevent the bathroom fan from ventilating that space adequately.
If you have a completely enclosed shower area, the bathroom fan will also need to be GFCI connected. Pretty much all water closets that I inspect have a bathroom fan with a light as well. Since the enclosed area is usually such a small area, going with a small 50-cfm bathroom fan is standard.
And if you absolutely don’t want to install a secondary fan, it’s recommended to leave the door at least partially open throughout the day. This allows the primary bathroom exhaust fan an opportunity to do its job.
Frequently Asked Questions
Here are a few frequently asked questions that are related to sizing a bathroom fan:
Can A Bathroom Fan Be Too Strong?
The short answer is yes. If the bathroom fan is significantly stronger than the recommended CFM, there is a chance it will cause backdrafting in the home. Backdrafting occurs when there are gas appliances in the home, and the powerful fan creates negative air pressure.
The negative air pressure can pull back in noxious gases back into the home from the gas appliances (instead of going outside) such as hazardous carbon monoxide. However, if you just increase the CFM 10-25% from the standard CFM recommendations, it should not be problem.
Can The Duct Size Affect CFM?
The quality of your duct absolutely can affect the CFM output of your bathroom fan. New bath fans will tell you the vent pipe size (diameter) that needs to be used such as 3-inch, 4-inch, or 6-inch. The best ducts for bathroom fans also tend to be pre-insulated which helps reduce noise and prevents condensation.
If the bathroom fan is rated for 130-cfm with a 6-inch duct, but you only have 3-inch installed, it will decrease the CFM performance. In addition to the duct size, the number of bends, overall length, and metal vs. foil type of duct can have an impact on CFM.
What If I Am Concerned About Noise?
Bathroom fans come with a noise rating called sones. Anything that is 1.0 sone or less is considered quiet. In fact, a bathroom fan with a 0.3 sone rating can be difficult to know that it's even on which is why they have LED indicator lights. A bathroom fan that is 3-4 sones is pretty loud. My guide on bathroom fan sones details how it works.
A higher CFM bath fan will likely have a higher sone rating, but not all. Higher quality bathroom fans tend to also have premium motors (variable speed and ECM) that can significantly reduce noise.
Are There Bathroom Fans With Difference Speeds Or CFMs?
A lot of the bath fans made by Panasonic have a speed selector feature which means you can choose between three different CFMs such as 50/80/110. There is a little switch on the housing that allows you to choose which CFM you want.
There are also bathroom fans with variable speed motors that start off on a slow speed when you first turn on the fan, and if it detects high 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 best bathroom fans with humidity sensors here.