What is CFM?

what is cfrm
(Last Updated On: August 25, 2020)

CFM is an abbreviation that stands for Cubic Feet per Minute. By definition, it is velocity measurement whereby air can flow into space or out of it. CFM helps to describe the ability of heating, air conditioning systems and ventilation. 

what is cfm cubic feet per minute

It tells the rate at which volume of air moves in a given period. For instance, In the case of a fan or blower, it shows how much air it can move every minute.

what is cfm
It is not easy to tell exactly how many CFM you need in an air compressor to maneuver your air tools. When the CFM is too little it means you won’t be able to continuously run your tools. On the other hand, too much of it means you have gone over the expected limit.


CFM and Air Compressors and Other Air Devices

Some air tools are constant demand appliances asking a high CFM. For instance, saws or pressure washers. Conversely, something like a nail gun may require a short burst of air from time to time, which might be perfect for a lower CFM.  

For that reason, to have a better understanding of how suitable a compressor is, it’s important to consider a combination of CFM and operating system to be certain you won’t feel let down.


How To Calculate The CFM Of Fan For A Room: CFM Airflow

CFM airflow is very important. If you want to buy a fan of your room, you have to do some mathematical process to figure out the room cubic area.

calculating cfm of a room

Also, you have to determine the rate you desire the air to circulate the entire room.

Step 1:

Measure the length and width of the room and then measure the height from the ceiling level to the floor.

Step 2:

After step1 measurement, multiply the width, length, and the height to get the cubic footage of the entire room. Let’s say the room is 10ft wide, 12ft long and 10ft high. So, it will be:

10 (width) X 12 (length) X 10 (Height) = 1200 Cubic feet

Step 3:

Now, you can multiply the number of times you would like the air to be exchanged with a cubic volume of the room. For instance, if you would like the air to be replaced twice for every single hour, then multiply 2 by 1200 which equals to 2400.

Step 4:

To calculate CFM, divide the answer you got from (step 3) by 60. So, you divide 2400/ 60 to get a CFM of 40. 

CFM calculation


What Is CFM In Bathroom Fans?

So remember now, CFM refers to the rate at which a given volume of air moves in a certain period of time. When it comes to a regular blower fan, CFM refers to the amount of air the fan can move per minute. Let’s take an example.

sizing bathroom fan cfm

CFM is commonly used to size bathroom fans. Let’s say you have a 1000 CFM fan. This means that the fan can remove all the air from a box of dimensions 10ft x 10ft x 10ft in one minute. If the fan is a 10000 CFM fan, then it can remove all the air from a box of 100ft x 10 ft x 10 ft in a minute and so on.

So how do you calculate CFM for bathroom fans?

The formula for calculating CFM is commonly used when installing a bathroom fan. Let’s take an example:

The Size of The Bathroom: Let’s say the fan is 12’ x 12’ and you have a 12’ ceiling. This will equal to 12 x 12 x 12 = 1728 cubic feet

Turnover Per Hour: Now let’s say you need to turn the air over fast, about 10 times per hour. In such a case, 17280 cubic feet of air will need to be moved per hour.

CFM: Now take the turnover and divide it by an hour – 17280 / 60 min = 1954.67 cfm

In this case, you can conclude that the bathroom fan should be 1954.67 cfm to turnover air at the required rate.


Continuous Vs. Discontinuous 

The way you use your tools matters a lot. If for instance when using an impact wrench, do you work continuously without letting go the trigger or you have short breaks during the work process to let the compressor to catch up? 

Air tools require a CFM that follows the manufacturer’s guide. Having an air compressor with (100%) duty cycle is the best if your application requires the air tools to run for a lengthy time to operate the tools continuously.

Conversely, tools used intermittently may get away with less powerful air compressor using an air tank receiver and may help you to save money in the process. 


Use of Multiple Tools

A very effective scenario that may impact CFM requirements is to utilize multiple tools at once. In such cases, it’s important that you understand what cfm is and are not aksing questions like what is cfm and why does cfm affect me. For you to speed up on a given task you can have two or more people work on a single job running multiple tools. Nonetheless, to achieve that, you need to combine the CFM requirement of every tool that will be running to get overall CFM requirement. 


Reducing the compressed air CFM

Asking the question of what is cfm is common because of its wide application in the compressed air industry. Sometimes you may wan to increase or decrease cfm. Here is an example of one of the questions our readers asked:


My compressor delivers about 90psi at 9 CFM.

I can set the gauge to 30psi, but how can I lower the 9cfm to 3cfm ?

Your answer is appreciated.




Hello Wolf:

That’s a tough one to answer simply, and often the solution is trial and error.

By setting your regulator to 30 PSI, you are reducing the flow through the regulator so that the downstream pressure never gets enough flow to allow that downstream line pressure to exceed that 30 PSI setting, give or take a few PSI depending on the quality of the regulator.

You are correct. That limits the flow somewhat, but really, the compressed air regulator is all about pressure, not flow.

In order to reduce the CFM to an acceptable level, you will need to restrict the air flow, and that’s commonly accomplished through the use of a needle valve. Not a flow control, but a Needle Valve. It can be a bit confusing, since a needle valve is a flow control device. If you follow the preceding link, you will get more information about these devices as they pertain to compressed air.

Inserting a needle valve into the air line to your application will both reduce the PSI temporarily until the line fills, and give you control over the ongoing  flow as your device draws air.

How will you know what the lower flow is? For that, you will need to install some sort of compressed air flow meter, unless there is something on your device that allows you to monitor the flow of air into it.


I Need 10 CFM @ 100 PSI

Here is an example of a question one of our readers asked. This question proves the importance of knowing what is cfm and ist applications.

Question: One of the visitors to my site asks the following:

“I am working on a model airplane and I need a compressor with about 10 CFM at about 100 PSI, as light as possible. Your product seems to have additional components such as the cover and gauge that are not needed for my purpose. Could you please advise me of what compressor I should get?”


Hello, and thanks for writing.

You can remove the gauge from your compressor should you not want it, but no compressor comes without one. The gauge shows what the downstream pressure will be from the regulator, which too is almost always a component that comes with every compressor.

You want the regulator so that you can lower the pressure coming from the compressor discharge to try to eliminate pulsations as the compressor turns on and off with air consumption and to use the absolute minimum of air pressure necessary for the application, reducing compressor run time and power consumption.

As far as the cover goes, different manufacturers cover the compressor head differently, but in most cases it’s not necessarily for appearance, but to keep persons from accidentally touching the compressor head, which can become quite hot when the compressor is running.

You say you need 10 CFM @ 100 PSI. Is that continuous, or for a few seconds at time?

That’s quite a bit of flow for a DIY type compressor. Frankly, it will be pushing it to get that with a 110 VAC compressor. Regardless of what horsepower the compressor is rated for, with 110 VAC in a 15 amp circuit, you will generate about a maximum of 2 HP of compressor power, and that might give you 8 CFM @ 90-100 PSI.

In order to get continuous 10 CFM you might have to go to a larger compressor and 220 VAC power.

So, until I have a better understanding of your needs I can’t be more specific about what compressor. If I can help further, write back.




How much air is 400 CFM?

That’s an interesting question.

To better understand the answer, first we’ll look at the concept of CFM and how it applies.  It might be better to ask, how many compressed air using devices can I run with 400 CFM?

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For example, if a compressed air drill uses 20 CFM of compressed air to run, then, in theory, you could run 20 drills at the same time with an air compressor that generates 400 CFM.

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Or, a big sand blast cabinet might use 200 CFM of air when parts are being sand blasted. If you have two of these sand blasters running at the same time, then you would use up all the air a 100 HP air compressor would deliver which is about 400 CFM.

For future guidance, typically, you will generate about 4 CFM of compressed air at 90 PSI per HP of electric compressor motor.


How much air is 400 CFM?

Is 400 CFM of compressed air at 90 PSI  the same as 4 CFM of air at 40 PSI?

That has to be answered as yes and no.

When using compressed air at 90 PSI, and the air tool uses 400 CFM to run, then you will need 400 CFM at 90 PSI to run it.

If an air tool needs 400 CFM of compressed air at 40 PSI, you still have to deliver the flow and the pressure you need, yet if you have a compressor capable of delivering 400 CFM at 90 PSI, and the compressor tank is full of air at 90 PSI, then that air will last longer when using it at the 40 PSI level.

It is not enough to ask how much air 400 CFM is, without knowing at what pressure that 400 CFM is being delivered at. If delivering compressed air at 400 CFM and at 10 PSI, you would not be able to properly run an air tool that needed 400 CFM of air at 90 PSI.

In order to ensure that your air compressor can deliver the compressed air you need, you will need to know the flow of air required in CFM and the pressure that flow has to be at for a specific air tool or air tools.


Compressor Free Air CFM

First off, it’s important that we both work with the same definition of CFM and correspondingly, SCFM. To get more information on either of those terms, start here.

Compressor Free Air CFM

Although there are scientific definitions of CFM and SCFM, it’s my experience that there are no standards when companies that manufacture compressors or air tools express the capabilities and flow requirements. Some use CFM, some use SCFM.

My definition is simple. If it’s pre-compressor, I use SCFM. If the compressed air is post-compressor, I use CFM.

In order to determine the size of a compressor required for a particular application, you will want to know the requirements of the application(s) at the PSI (pressure level) and the compressed air flow required by that application, at that pressure level,  in CFM. That’s the question to ask the compressor vendor.




We have often been asked to define CFM, so we hope you fully understand whats cfm stands for. So what is CFM? We are sure you can answer that question now. The most crucial detail when looking for a compressor be careful to note CFM rate as that will tell the number of tools you can use at a given time plus total power capacity. Every tool connected to a compressor has its CFM needs, so, it’s recommended to select an item with maximum CFM. 

Doing that, it allows your compressor to appropriately run the full array of tools in your store.

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