More than a few times we’ve seen, through folk’s comments, that they are struggling with the difference between PSI and CFM.
We’ll see if we can make it a bit easier on this page.
What is PSI?
Push down on your desk top with your index finger.
For sake of this discussion, let us assume that the area on the tip of your index finger is exactly one square inch in size. That being the case then the force that you were pushing down on the desk with could be expressed as so much force pushing on that one square inch. Right?
Now, let’s pretend that you’ve got a meter on your arm that shows you arm muscle force. Push down on your index finger until that arm force meter shows 10 lbs. of push.
At this time you are now exerting 10 lbs. of push (force) on that one square inch of index finger onto the desk. Does this make sense?
Since your finger tip is one square inch in size, and you are pushing down with 10 lbs. of push (force), you are exerting 10 PSI.
PSI is a short form for:
P = Pressure
S = per Square
I = Inch
You can see the PSI displayed on your air compressor in two places. One is the tank gauge, which displays the tank pressure in PSI. The other is the gauge on the regulator on your air compressor, which displays the pressure setting of that regulator in PSI.
The Difference Between PSI And CFM
We hope you understand what PSI means now. What does CFM mean, then, and how do they work together?
CFM is a measure of flow. As it relates to your air compressor, that air compressor has an output measured in (usually) CFM.
CFM is a short form for:
C = Cubic
F = Feet
M = per Minute
Your air compressor has an output flow, that is to say, that it is designed to produce so many CFM.
A smaller, lower HP air compressor can perhaps only deliver 2 CFM of air. A 200 HP monster compressor could deliver 800 CFM of air.
Flow at What Pressure?
The easiest way to understand how PSI and CFM work together is to read the manual of an air tool. Here is part of a page of a manual from a Husky air tool die grinder.
This particular air tool is designed to operate at 90 PSI-G. The G refers to gauge pressure, the pressure display on a typical air gauge.
Elsewhere in this manual it will indicate that this die grinder needs that 90 PSI at a specific flow… that being, for example, 23 CFM.
To drive this die grinder then your compressor will need to deliver 23 CFM of compressed air at 90 PSI.
The manual will say something like “23 CFM full load”.
Does Compressor Have Enough CFM?
You can relate this information back to your air compressor by understanding that air compressors under 10 HP typically deliver 3-4 CFM of compressed air at 90 PSI for every HP of electric motor size.
This Die Grinder
It needs 90 PSI. We now know that this is a measure of force.
It needs 23 CFM. We now know that this means a measure of compressed air flow.
The compressor to run this die grinder, and keep up with the demand of this tool, will have to have an electric motor of around 6 HP.
Other factors come into play to help determine if your air compressor is big enough.
- how long per use of this die grinder
- how big the compressed air tank
- whether the die grinder use can be discontinued while the compressor recharges
Knowing the difference between PSI and CFM is necessary to ensure that you will not be disappointed in the inability of a specific air tool to do the job required, and whether or not you have the compressor with the capacity to actually drive a specific air tool.
Got a ?… I have an air compressor that is rated at 155 psi. And 5.1cfm. The tool I just purchased requires 125psi @ 7cfm. Will my compressor run the tool?
Might, but doubt it if you are trying to run the tool for any length of time in one go. I have not been able to find a formula to tell me the correlation between pressure and flow, for example, if a compressor flows 5.1 CFM at 155, how much will it put out at 125 PSI so cannot calculate it for you.
I also, like I don’t believe the mileage estimates on cars, don’t believe the published capacities of compressors. I’d be surprised if your compressor really generated the 5.1 CFM at 155 PSI it claims as you will be using the compressor in ‘real world’ environments and not the factory ‘ideal’ situation.
Depending on your need for the air tool, as in you are planning on making money with it, I suspect your compressor will be too small.