Many folks refer to motor horsepower and tank capacity when describing or sizing an air compressor. While these specifications are nice, consider the compressor as simply an “air pump” and it is more important to rate the ability of that “pump” to do a job. There's a common air volume requirement rating for evaluating the ability of a compressor, air tool or spray gun to perform a certain job. That is the “cubic feet per minute” or “cfm” rating. As an example; this would be listed in specs as “12 cfm @ 85 psi”. This translates into a device requiring an air volume of 12 cubic feet of air per minute at a pressure of 85 pounds per square inch.
The determining factor in choosing a compressor is this cfm. Consider what your typical cfm demands will be by checking the cfm specs on your air tools, blaster, spray guns, etc. Be sure to choose a compressor that's capable of delivering sufficient cfm, at the rated psi, that will supply the highest air volume using the tool you have or will require. Using air tools that exceed the cfm rating of the compressor will cause the compressor to run constantly, overworking and overheating it, resulting in permanent damage.
This is also very important when choosing a tool, blasting equipment or paint gun. You want to be certain the air-operated tool's requirements do not exceed the capabilities of your existing compressor. For example; if you have a compressor with a maximum output of 9 cfm @ 90 psi and you are considering a blast cabinet requiring 12 cfm @ 85 psi, the compressor will not be up to the task and the blast cabinet will not perform properly. This is also true of HVLP paint guns and other air-operated tools.
In short, it's always best to find the highest cfm requirement of all the tools you own or ever plan to acquire, then add 25% to that figure when choosing a compressor. I always use 85-95 psi as a baseline.
Some Other Considerations
How many people are in the shop? – Of course if you have 4 guys using air at the same time you need to add up their needs. Use the rule of thumb above, but you can round down some, since it’s not likely all 4 will be using the air continuously with their most air hungry tools.
What quality of air do you need? – If you are setting up a shop with a paint spray booth or for powder coating, the extra cost of a compressor with built in aftercooler and air dryer is totally worth it. Most air tools are not that picky about clean dry air, but just a little oil or water in the air supply can ruin a paint job that took hours to spray and days to prep. If you don't want to shell out the extra cash to buy a compressor with these functions built in you can always add them on to your current compressor.
How loud will it be? – Big reciprocating piston compressors are loud, so take that into account when shopping and deciding where in the shop you want it. Buying a compressor with a tank larger than you need means the compressor doesn’t have to run as often, which is great if you have to work right next to it.
Single or 2 stage? – Depending on what you are using the air for you may want the higher pressure of a 2 stage compressor. Single stage compressors tend to produce a greater volume of air, but not at the pressure a 2 stage can attain. Painting with a HVLP gun requires a greater volume of air, while most air tools run on a greater pressure.
110v, or 220v powered? – Chances are you will be using 110 or 220 volt power. If your shop is wired for 220v power, go for it, it will cost you less in electricity and the compressor is likely professional quality or close to it. All portable compressors are 110v, but 110v shop compressors do exist if you can’t justify rewiring the whole shop just to power a compressor.