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Choosing the right size
air compressor

The right size air compressor will meet your needs in two essential areas: air flow and pressure. It's important get a compressor that meets your needs in both areas – otherwise you risk damage to both your tools and the compressor itself.

Air pressure requirements (PSI)

Of the two, pressure is the more straightforward consideration. In the U.S., pressure is usually measured in PSI, pounds per square inch. (European manufactures often use the metric pressure measurement "bars.") The tool or piece of equipment you intend to power that has the highest PSI requirements will set your minimum: if you intend to use your compressor to run several hand tools that require 50 PSI and a sprayer that needs 90, your absolute minimum PSI requirement is 90 PSI.

As with most machinery purchases, buying a piece of equipment that just matches your minimum specifications isn't a great idea. choosing a compressor capable of supplying slightly higher pressure than you need gives you a safety margin – if your minimum is 45 PSI, look for a compressor that offers 50 or 55 PSI.

Airflow requirements (CFM)

Sizing up the airflow you need is slightly more complicated, because instead of just choosing the highest rating among your tools, you need to add them up. If you expect to simultaneously use 3 hand tools that require 50 CFM (cubic feet per minute) each, your compressor has to be capable of 150 CFM.

Building in a safety margin is even more important with airflow than it is with pressure. The recommended approach to setting your CFM target is this: collect the CFM requirements for all the tools or equipment you want to be able to use at the same time. Add the requirements up, then multiply by 1.5. This gives you a figure that accounts for some bleed or inefficiencies in your setup.

Once you know these two figures, you're ready to start talking to air compressor dealers. When you do, make sure that the compressors you're looking at deliver both your requirements: the spec to look for "xxx CFM at xx PSI." Some compressors can alternate between delivering higher pressure at lower airflow, and higher airflow at lower pressure. Make sure the model you choose delivers the volume of air you need at the right pressure.

Other specifications

You may hear a lot about horsepower when buying compressors, but don't pay it too much attention. Horsepower is purely a measure of engine output, which doesn't always correlate exactly to the volume or pressure of the resulting air.

If you're buying a reciprocating compressor, consider the size of the air holding tank. By storing more air, a larger tank means that the compressor needs to run less often, helping you avoid overworking it or exceeding the duty cycle.