Air compressors come in different sizes, and variations. There are different horsepower motors, different size tanks, different piston configurations, but perhaps more importantly, all of them are rated in cubic feet per minute, or CFM of compressed air.
Typically a compressor is rated at a certain number of CFM at a specified pressure. The portable 30 gallon Eastwood Compressor that is our best seller (#31436) is rated at 5.7 CFM at 90 PSI (pounds per square inch), while its 80 gallon bigger brother (#31440) is rated at 22.1 CFM at 100 PSI. Meanwhile our Straight Line Sander (#13747) is rated at 5 CFM at 90 PSI, while our 50-lb. Pressure Abrasive Blaster (#51117) needs 10 CFM at 90 PSI, and the popular Devilbiss Finishline HVLP spray gun (#12826) needs 13 CFM at 23 PSI. So that gives you some frame of reference.
The general stated CFM rating of a compressor should only be used as a guide line. Typically they are using DCFM or displaced CFM, which uses math to calculate how much air the cylinder of the compressor will be pumping out for a given RPM. This is kind of like the old style horsepower ratings of the 1960 which didn’t take into account a lot of real world factors. In the real world the compressor loses flow due to heat and friction, as well as any bottle necks in the fitting between the cylinder and the tank (and your air tool, but they can’t be responsible for your shop’s plumbing). There is also the ACFM or actual CFM which is taken by running the compressor and measuring the actual amount of air coming out of it.
SCFM stands for standard CFM and is a more scientific way of stating things. SCFM usually starts with the measured CFM flow of the compressor, or ACFM, then adjusts it for a set of standard conditions agreed upon by science and industry. These are 14.5 PSI atmospheric pressure, 68 degrees F, and 0% relative humidity. If you think about it this makes sense because if the ambient pressure is lower (in Denver for instance) there is less air around to compress so the ACFM would be lower. Of course the hotter the ambient temperature is the less dense the air is, we all know that from tuning carbs at the drag strip. And more humidity means more water vapor in the air, which when compressed turns into water and reduces the air output.
Just because a compressor is rated for a certain CFM at 100 PSI does not mean it produces a significantly greater CFM at 90 PSI. The piston displaces the same amount on each stroke, and the RPM of the compressor motor is constant, so at a lower PSI there is less loss from heat and parasitic factors like friction, but not really a lot more air being pumped.
A good rule of thumb when buying a compressor is to look for one rated at 1.5 times the CFM requirement of the tools you are likely to use continuously for a long period of time.