A bathroom exhaust fan is an essential part of every bathroom. Picking exactly the right fan for your bathroom starts with sizing. Namely, you want to figure out what size bathroom fan do you need.
Adequately venting your bathroom is needed to reduce the moisture levels. This will prevent mold and mildew growth.
The size of bathroom exhaust fans is measured by the airflow they can achieve. This airflow is measured in CFM or Cubic Feet per Minute. CFM is simply a unit that measures the amount of airflow (it stands for Cubic Feet per Minute).
Now, as we see, when you’re sizing a bathroom fan, you’re really asking this specific question:
“How many CFM for bathroom fan?”
Bigger bathrooms need more CFM fans, smaller bathrooms need a less powerful fan.
Example: A 50 CFM bathroom fan is adequately sized to be installed in a 47 sq ft bathroom. A 150 CFM bathroom fan can handle a 141 sq ft bathroom. We presumed an 8 ft ceiling in both of these calculations.
How do you calculate the CFM needed for a bathroom fan? The great thing about sizing a bathroom fan is that it’s fairly simple. According to HVAC rules of thumb, you only have to follow 1 rule:
The bathroom fan should achieve about 8 ACH (Air Changes per Hour) worth of CFM.
We’ll explain in full what ACH actually means. The key here is that this simple rule dictates that you need about 1 CFM per sq ft of the bathroom for proper ventilation (8 foot ceilings).
We will show you how to calculate how many CFM bathroom fan you need.
To help everybody out, we have also created a ‘Bathroom Fan CFM Calculator’. You simply input the size of your bathroom in square feet and ceiling height, and the calculator will dynamically determine how many CFM bathroom fan you need.
You can find the bathroom fan CFM calculator further on, complete with the ‘Bathroom fan CFM chart’. The chart includes calculated exhaust fan CFMs for all bathrooms (from the smallest 20 sq ft bathroom to the biggest 500 sq ft bathroom).
First of all, however, let’s look into how we determine how many CFM bathroom exhaust fan you need in a step-by-step process:
How To Determine What Size Bathroom Fan You Need?
Note: This is how you can calculate CFM needed for the bathroom manually (by hand). Of course, the much easier way is to use the bathroom exhaust fan size calculator (in CFM) below, or consult the bathroom fan CFM chart below the calculator.
To operate properly, any bathroom exhaust fan has to have sufficient power to change all the air in a bathroom 8 times per hour. That’s what 8 ACH means. You’re changing all the air every 7.5 minutes in order to reduce humidity levels when showering, for example.
We’ll explain the calculation first, and summarize the calculation in the shorter step-by-step process at the end of this chapter:
Calculating Bathroom Fan CFM By Hand
Bathroom exhaust fans can produce anywhere from 40 CFM to 350 CFM of airflow. That is the very airflow that needs to be sufficient to ventilate your bathroom 8 times per hour.
Example for a 100 CFM bathroom fan: Every minute, such a fan generates 100 cubic feet of airflow (100 CFM). That means that in 1 hour, a 100 CFM bathroom fan will generate 6,000 cubic feet of airflow (100 CFM × 60 min = 6,000 cubic feet).
Now, this 6,000 cubic feet of airflow has to change all the room in the bathroom 8 times. That means we need to figure out how much air is in your bathroom.
We can do that by calculating the volume of air in the bathroom. For this, we will need to know 2 factors, namely:
- Bathroom square footage (floor area). A 10×6 bathroom has 60 sq ft, an 8×8 bathroom has 64 sq ft, and a 10×10 bathroom has 100 sq ft. You know how this works.
- Bathroom ceiling height. The standard bathroom ceiling height is 8 ft. All the calculations you will find in the bathroom fan CFM chart further on use 8 ft. Of course, some bathrooms can have 10 ft or even 12 ft ceiling (high ceilings). That means more air we need to take into account.
Here is how the volume of air in the bathroom is calculated:
Volume Of Air (In Bathroom) = Square Footage × Ceiling Height
Example: How much air is in an 8×10 bathroom with an 8 ft ceiling?
Volume Of Air (8×10 With 8 Ft Ceiling) = 80 Sq Ft × 8 Ft = 640 Ft3
In this example, we need a fan that will change 640 cubic feet of air 8 times per hour (it will vent through the exhaust fan). How minimum CFM should such a bathroom fan produce?
In 1 hour, the total airflow should be 640 ft3 × 8 = 5,120 ft3. But as we know, the airflow of bathroom exhaust fans is not expressed in cubic feet per hour (CFH). They are expressed in CFM or cubic feet per minute.
To convert CFH to CFM, we need to divide that 5,120 ft3 by 60 (because 1 hour = 60 minutes). Here’s how we can finally get the bathroom fan airflow (in CFM):
Bathroom Fan Airflow (8×10 With 8 Ft Ceiling) = 5,120 ft3 ÷ 60 min = 85 CFM
That means that this 8×10 bathroom with an 8 ft ceiling will require an 85 CFM fan.
Note: You can see that this is an 80 sq ft bathroom and it needs an 85 CFM fan. That’s very close to 1 CFM per sq ft, right? If you have an 8 ft bathroom ceiling, you can use the 1 CFM per square foot rule of thumb without much loss in accuracy. If the ceiling is higher (or lower), you can’t use this rule of thumb.
Another note: If you have large items in the bathroom (large toilet, bathtub, jetted tub, washing machine, etc.), you can deduct that from the overall total air calculation. You don’t have to include smaller items like fixtures.
Let’s summarize all these calculations in a simpler step-by-step manner:
Short Step-By-Step Summary Of Bathroom CFM Calculation
- Measure bathroom length, width, and ceiling height. Example: 8×10 bathroom with 8 ft ceiling.
- Calculate the volume of air in the bathroom by multiplying length × width × height. Example: 8ft × 10 ft × 8 ft = 640 ft3.
- Multiply the volume of the room by 8 (due to the 8 ACH rule of thumb). Example: 640 ft3 × 8 = 5,120 ft3.
- To calculate bathroom fan CFM, divide the result by 60 (due to 1h = 60 min). Example: 5,120 ft3 ÷ 60 = 85 CFM.
Hopefully, this will help you to figure out how much CFM you need for your bathroom.
We do understand this whole process might be a bit complex. That’s why we have designed the bathroom exhaust fan CFM calculator that simplifies this process:
Bathroom Fan CFM Calculator
Just input the square footage of your bathroom and ceiling height, and the bathroom ventilation fan size calculator will dynamically calculate the CFM needed (you can slide from left to right to get the feeling of how CFM changes depending on the size of the bathroom or ceiling height):
Using this calculator, you can determine what size bathroom fan you need. To make things even easier, we have calculated fan CFM requirements for the smallest 20 sq ft bathrooms right up to the biggest 500 sq ft bathroom.
You can find all the calculations in this bathroom fan size chart:
Bathroom Fan CFM Chart
| Bathroom Size (Sq Ft): | Required CFM: |
| 20 sq ft | 21 CFM |
| 30 sq ft | 32 CFM |
| 40 sq ft | 43 CFM |
| 50 sq ft | 53 CFM |
| 60 sq ft | 64 CFM |
| 70 sq ft | 75 CFM |
| 80 sq ft | 85 CFM |
| 90 sq ft | 96 CFM |
| 100 sq ft | 107 CFM |
| 120 sq ft | 128 CFM |
| 140 sq ft | 149 CFM |
| 150 sq ft | 160 CFM |
| 160 sq ft | 171 CFM |
| 180 sq ft | 192 CFM |
| 200 sq ft | 213 CFM |
| 220 sq ft | 235 CFM |
| 240 sq ft | 256 CFM |
| 250 sq ft | 267 CFM |
| 260 sq ft | 277 CFM |
| 280 sq ft | 299 CFM |
| 300 sq ft | 320 CFM |
| 400 sq ft | 427 CFM |
| 500 sq ft | 533 CFM |
Keep in mind that big bathroom exhaust fans – with a high CFM rating – can produce quite a lot of noise (sound levels are measured in sones). You can check the quietest bathroom exhaust fans here.
We hope you found all of this helpful. You can also check a universal CFM calculator here; it might be helpful in some cases.
If you have any questions about the calculations or would like our help in determining the CFM, you can comment below and we’ll try to help you out as best we can.
Thank you.
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This is a really well thought out, and clear explanation of calculating these CFM values based on room size. As someone who studied physics, I appreciate the effort that went into this article.
From a fellow physicist, thank you 🙂
Hello, How do I size a inline bath vent fan that will vent 2 bathrooms (one vent fan for 2 bathrooms)
Thank you.
Hi Tom, the easiest thing would be to go with the 1 sq ft = 1 CFM rule of thumb. If you have two 50 sq ft bathrooms each, you would need a 100 CFM bathroom fan. This is a good logic to follow.