Measuring Compressed Air Flow

SCFM, the measurement of free air flow. Confusion with SCFM, over and above the compressors themselves, is an issue that draws a steady stream of visitors to this air compressor information website. I hope the information on this page helps clear any confusion about SCFM that you may have.

I am not too surprised at the confusion. Even though I think I understand the difference between SCFM, CFM, ACFM and other acronyms that have to do with the measurement of compressed air flow, my definition may not be the one that any specific compressor vendor is using to provide flow characteristics for their machine. It also might vary from the definitions of the air cylinder, or air valve sales rep.

The definition of SCFM is suspect due to the changing parameters from different sources. And it is how that definition applies to sizing a compressor that complicates the issue with many folks.


Compressor discharge flow measurement is confusing and fraught with error for the person that is not dealing with these issues regularly. Heck, even if you are involved with fluid power, applying SCFM in a meaningful way to compressed air flow is confusing.

I have canvassed a variety of sources and have found that even those that are considered most reputable have differing definitions of SCFM, and how that definition is applied for us compressed air users.

SCFM Has Rigid Circumstances

In order for specific air flow to be measured in SCFM, most experts agree that the air flow must meet the following set of rigid environmental circumstances;

  • The air temperature must be 68 degrees Fahrenheit
  • The relative humidity of the air must be 36%
  • The air itself must be measured at sea level

Having said that, some other sources indicate that the relative humidity of the air must be zero in order for the air flow to be measured in SCFM. Another source says that the R.H. must be at 50%, for the flow to be rated as SCFM.

Confusing? You bet!

Consider this! Few of us have plant locations exactly at sea level where the ambient air pressure is 14.7 PSI. We have little control over the exact air temperature or the relative humidity in our plant. As air exits the compressor discharge it will always be a lot hotter than 68 degrees F. That compressed air will have a much higher humidity level than the 36% (or the zero, or the 50%) that is specified for an SCFM rating.

Simple Definition Of SCFM

The acronym SCFM stands for Standard Cubic Feet of air per Minute.

If you need more than a simple definition, if you are trying to understand how that term applies to measuring compressed air, then please read on.

An Airflow Yardstick

Some compressor manufacturers use SCFM as their yardstick to measure free air flow into the intake filter on an air compressor, not the flow of compressed air from the discharge of their compressor.

Some compressor manufacturers will use the term CFM for the flow of air out of their compressor discharge. Others will use the acronym ACFM (Actual CFM) as the measure for air from their compressors. Another might use ACFM as "after-compressor feet per minute".

And some manufacturers will use the acronym SCFM as their measurement of air from the discharge port on their compressors.

For us then, SCFM must be considered as a theoretical measure of flow that does not have a lot of impact on us when it comes to determining the compressed air flow needed to do work.

Did you know that...

Air leaks can be found by listening for them, using the tried and true method of soapy water brushed onto joints, or via an ultrasonic leak detector. Ultrasonic leak detectors can find leaks even in a noisy plant environment, and by using ultrasonics, you do not have to worry about cleaning up the soapy water when you are done testing!