What is CFM?
CFM is the usual unit of measure for the volume of discharge air from a compressor over time. As you may have guessed, it’s units are based on; volume / time.
What does CFM mean?
CFM is the acronym for Cubic Feet per Minute. A compressor has a specific number of cubic feet of compressed air per minute (CFM) of flow from its discharge port.
When it comes to using compressed air in your plant or home workshop, you will want to know how many cubic feet per minute you can expect from the discharge port of your compressor. This will help determine if that air compressor has sufficient compressed airflow to power your air tools or other air-consuming applications.
To do that you need to know what CFM a particular device or tool will require to function within its design parameters. The device or tool will require a certain number of CFM at a specific air pressure, to work properly.
Air Compressor HP to CFM Calculation (rule of thumb)
A rule of thumb is that each 1 HP motor size generates about 3-4 CFM of compressed air flow at about 90 PSI.
CFM to HP – Rule of thumb
Or the equivalent of 1 CFM to 0.25-0.33 HP at 90 PSI.
Air Compressor kW to CFM Calculation (rule of thumb)
A rule of thumb is that each 1 kW of compressor power generates about 4-5.5 CFM of compressed air flow at about 90 PSI.
Or the equivalent of 1 CFM to 0.18-0.25 kW at 90 PSI
An exception to the rule of thumb
This air production by each HP of motor size is pretty much an industry standard, though it doesn’t apply as accurately to compressors under 10 HP. For compressors smaller than 10 HP, you will need to read the specifications for that particular unit to determine their exact flow and pressure rate capabilities or use the “guestimate” of 2-3 CFM of discharge flow at 90 PSI, per HP of electric motor.
When you have sized all of your applications and totaled up all of the air you’re going to need now and for any prospective plant or air demand increases, and you are out searching for the right air compressor, you would divide the number of CFM you need by 4, and that will give you a rough idea of the horsepower rating of the compressor required.
What about S-CFM?
SCFM is another unit of measure you’ll be familiar with in the world of compressed air and in particular, compressed air tools. You should take a look at my article on the difference between SCFM and CFM to enhance your understanding in this area – it’s a bit beyond the scope of this article!
Differences between manufacturers stated CFM
Not all compressor manufacturers rate their compressor output the same way. You might see a compressor showing a discharge rate at what appears to be an acceptable CFM, but on closer inspection find that the figure is predicated on a much lower pressure than you might need.
Discharge rates in CFM at higher pressures are always quite a bit lower than discharge rates at lower pressures, for that same compressor.
Ensure that the unit you select will give you both the CFM you need, and the pressure your equipment demands to work properly for you.
Questions about Air Compressor CFM
Question: Does 2 hp compressor equal 8 cfm?
by Clive Haylock
(Sacramento, Ca, USA)
Hello I have been reading through your site trying to find information that will help me purchase the right air compressor.. from reading your info I think that I might understand it but i just want some clarification..
I am buying an air ratchet that requires 5.5 [email protected] 90psi… you stated that the rule of thumb is 4 [email protected] 90 psi for every hp.. so it does that mean if i buy a 2hp compressor should i assume that it will provide me [email protected] per the rule of thumb? for example i have seen some compressors with the specs on the unit that states 3.5 [email protected] 90psi and 5.0 [email protected] 40psi and this has a motor with 1.5hp…inclosing, is there a calculation difference between scfm and cfm, i didn’t see it in your postings..
My Response: Hello Clive. Glad to see that you’ve been reading the info on the site. That’s gratifying.
Yes, the rule of thumb is that you will get about 4 CFM of compressed air for every HP of electric motor.
I also say in many places that this rule applies to compressors over 10 HP. Why? Because they will typically be powered by at least 220 Volts or more.
The compressor converts electrical energy into stored energy. That conversion process uses the electricity that’s coming from the plug. In the typical DIY shop, that plug will have 110 Volts.
There isn’t enough available energy in a 110 Volt energy supply to produce the compressed air capacity of 4 CFM per 1HP. If you were running 220, you would get that, approximately.
Question: COMPRESSOR HP / and compressor flow
what would be motor rating of a compressor capable to deliver 25 cfm at 12 psi
I keep looking, but I have not found a formula that will allow me to convert the flow of a compressor at 90 PSI to the flow of that same compressor at 12 PSI, or any other number.
My Response: All I can tell you is that you should get around 4 CFM of compressed air flow at around 90 PSI for each HP of compressor motor.
What that same flow would be at 12 PSI, I can’t figure out. It would be higher, but how much… anyone out there know?