Air at greater than atmospheric pressure- usually 100 pounds over atmospheric pressure- or 100 PSIG. P.S.I.G means pounds per square inch gauge. Air which has been compressed above atmospheric pressure will return to atmospheric pressure when released. Therefore; compressed air is STORED ENERGY! The amount of this stored energy, or the ability of the compressor to supply this energy is expressed in both CFM and PSIG.
CFM or Cubic Feet Per Minute is the volume of air measuring the compressors capabilities. A good rule of thumb is one HP will provide about 4 cubic feet of compressed air at 100 PSIG.
Example: 5 HP Compressor will supply 20 CFM at 100 PSIG
100 HP Compressor will supply 1,000 CFM at 100 PSIG