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How to Calculate CFM

Written by Michael Cheng

Published on August 18, 2022


How to Calculate CFM

Learn how to calculate CFM when choosing a fan or air conditioning system for your home.

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Choosing any type of air circulation system for your home, such as a fan or HVAC unit, starts with understanding cubic feet per minute (CFM) ratings. CFM is used to determine how effectively your fan can keep your room cool.

But that’s not all. Some rooms inside your home, like your bathroom, need a specific CFM rating to prevent mold and mildew from growing on surfaces. Using a properly rated air system can also reduce surface degradation and can help furniture last longer!

Now that you know why calculating for CFM is important, let’s dive into the world of CFM ratings and their applications in choosing the right fan for your room or home.

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What is CFM?

CFM is the measurement of airflow or the volume of air in cubic units. This is used to gauge the amount of air a fan or other similar types of equipment, such as HVAC systems, bathroom exhaust fans, ceiling fans, air purifiers, blowers, and more can move inside a room. CFM is also applicable to non-cooling systems, like heaters, British Thermal Unit (BTU) ratings for stoves, kitchen range hoods, and air compressors.

The higher the CFM rating of a fan, the faster it can circulate air within a space. Fans with high CFM ratings are more effective in promoting airflow than those with low CFM ratings.

The CFM rating you need varies depending on the size of the room, the number of preferred air changes per hour (ACH), and the activity or application. ACH refers to the number of times the volume of air inside a room is removed or replaced every hour. The number of required ACH varies, depending on the type of room or space. For example, kitchens require 7-8 ACH. Basements need much less, at 3-4 ACH, because the space is rarely occupied. 

How to calculate CFM

To calculate CFM, you first need to gather the following information: room size (square feet of the room), the ceiling height (in feet), and the number of required ACH. 

Next, use the following formula to calculate CFM: 

CFM = Area x Height x ACH / 60

In application, let’s say you have a 200-square-foot room with a 10-foot ceiling and you need four ACH.

Plugging in all of the information into the above CFM formula would look like this:

CFM = 200 square feet x 10 feet x 4 ACH / 60

CFM = 8,000 / 60

CFM = 133.33 or 134 (rounded up)

Now that you know how to compute for CFM, let’s see how different ACH requirements of rooms affect the CFM ratings of fans.

ACH requirements for specific rooms and living spaces

ACH is a major factor in calculating CFM. In fact, just changing the ACH will result in a different CFM rating! The higher the ACH, the higher the CFM rating and vice versa. As an example, let’s revisit our previous equation above. We used four as the required ACH to arrive at 134 CFM. What happens when we double the ACH to eight?

The CFM rating changes to 267. Consider that the size of the room stayed constant; it is only the required ACH that changed. This is why you can have two rooms of the same size, but have different CFM requirements based on their ACH.

Below are various ACH requirements for rooms inside homes:

  • Basements: 3-4
  • Bedrooms: 5-6
  • Living rooms: 6-8
  • Kitchens: 7-8
  • Laundry rooms: 8-9

Why is CFM important?

Promotes comfort

Higher airflow and more ACH translate to a comfortable, cool room. If you’re wondering why some rooms feel like they have fresh air, it is mainly due to increased air circulation. Additionally, rooms feel cleaner and less sticky when more air passes through the area. 

Reduces humidity in bathrooms

Bathrooms tend to accumulate humidity when air circulation or ACH is insufficient. This is very apparent in bathrooms with showers. Ventilation fans with high CFM ratings remove humidity from bathrooms by actively pulling warm, humid air out of the space. The best part is that this works even in bathrooms without windows! Low humidity in bathrooms helps prevent mold and mildew from growing on surfaces, as well as reduces odors left by stale air. 

Improves lifespan of surfaces and furniture

Moisture can damage surfaces, paint finishes, furniture, and equipment with sensitive materials around your home over time. A properly rated air conditioner or fan reduces moisture by keeping rooms dry and fresh. 

Get CFM right, sleep better at night

Choosing a fan with a proper CFM rating for a room is essential for comfort and air quality, as it can help maintain a fresh living area. Finding CFM when buying or installing a fan is a straightforward process when using the following calculation formula: Area x Height x ACH / 60. High CFM fans generally consume more energy than low CFM fans; therefore, make sure you stick to your CFM requirement as closely as possible. You don’t need to exceed the required CFM rating of your room or home, as it can contribute to higher power consumption.

Don’t forget that rooms have different ACH requirements depending on what the space is used for, which will alter the CFM rating of the air unit. Busy spaces used for activities that can change the general environment, such as kitchens and bathrooms, usually have higher ACH and CFM requirements. On the other hand, rooms with minimal activity, including bedrooms, have moderate to low ACH and CFM requirements.

Lastly, fans are usually only one component of a large air system. Other parts of the system, such as ductwork, can also affect the efficiency of the air unit. Take these factors into consideration to ensure you have a fully functioning cooling system at home.

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Written by

Michael Cheng Content Writer

Michael is a content writer at Fixr.com. He has more than six years of experience in industrial manufacturing and legal documentation, covering electrical safety, large-scale infrastructure and maritime legislation.