Knowing your application is the first step towards choosing the right air compressor. Most tools and machines powered by compressed air will give you two important pieces of information: the required air pressure and air flow. The air pressure will usually be measured in PSI - pounds per square inch. Air flow is typically measured in CFM - cubic feet per minute. Each piece of equipment will have requirements for both: "3 CFM at 90 PSI," for example. You can run multiple tools off of one compressor, too, so gather this information for every piece of equipment that will be run off the compressor.

If your equipment is old or lacking documentation, you might not be able to get these exact figures. At least take down the make and model information - your compressed air dealer will probably be able to make an educated guess based on the type of equipment.

Once you've gathered the requirements for each tool, add up the CFM figures for all the tools that will be used simultaneously - that's the absolute minimum CFM to look for. The rule of thumb is to multiply that number by 1.5. This will give you a good margin of error in case some tools leak small amounts of air and keep you from overworking your compressor.

Next, take the single highest PSI requirement from any tool, and bump that up slightly as well. If your tools require 90 PSI, buying an air compressor that provides the CFM you need at 100 PSI will make sure you always have enough pressure to run your equipment. Don't overdo it, though: running a compressor at 120 or 150 PSI when you only need 90 is a waste of electricity.

Once you have both these figures, choose an air compressor than can meet the CFM and PSI requirements at the same time. Some dealers will list CFM ratings at 45 PSI, then tell you that the same compressor is capable of 90 PSI. It may be - but it might not be able to provide the same air flow at that pressure. Don't hesitate to describe exactly what tools you'll be running and ask the dealer point-blank if the compressor is adequate for the job - then compare his answers to the written specs.

You may see some compressors rated in ACFM (actual cubic feet per minute) or ICFM (inlet cubic feet per minute) - they're essentially the same measurements as CFM.

Compressor types and typical ratings
Knowing your requirements can help you decide which type of compressor is best for you. The largest two-stage reciprocating compressors max out at around 100 CFM, but in practice it's uncommon to see them over 50 CFM. Rotary compressors can reach 1500 CFM with a single stage and 2500 or more with two.

At the low end of the market, smaller air compressors - below 5 or HP - are almost exclusively piston-driven. And centrifugal compressors operate upwards of 1000 CFM and in a range of 400 to 8000 HP. At that size, centrifugal air compressors provide air at competitive costs.

Note that compressors are most commonly rated by horsepower, instead of CFM or PSI. A small home air compressor might produce 1.5 HP while large industrial compressors reach into the hundreds or even thousands of HP. The rule of thumb you can use to determine your approximate needs is that 1 HP should produce 4 to 5 CFM.

Don't worry too much about horsepower when you're shopping: more efficient compressors can produce better CFM with less horsepower, and manufacturers have different methods of rating HP. Instead, stick with the CFM and PSI specifications.

Plan for expansion
When evaluating your compressed air needs, be sure to consider your plans for the future. Air compressors can easily last 10 or 15 years, and while planning that far ahead is difficult at best, you should at least be able to plan for the next year or two. Will you be adding more tools? Or will you add more employees who'll be using the air simultaneously?

In addition, many businesses that introduce compressed air into their operations find additional applications that they hadn't anticipated: using it to clean up is a good example. While these applications may be less central to your business, they do increase demands on your compressor. Buying a compressor with enough capacity to handle your plans for growth is definitely the savvy business purchase.