When we talk about power in an air compressor, we usually speak in terms of horsepower, but there are many other ways to determine how much pressure a machine can provide. We use cubic feet per minute (CFM) to discuss the rate and volume by which a machine compresses the air. But the rate at which outside air enters the cylinder is influenced by heat, humidity and wind in the surrounding atmosphere. To consider these internal and external factors, manufacturers use standard cubic feet per minute (SCFM), which combines CFM with those outside factors of pressure and humidity. Another rating that you might see is displacement CFM, which looks at the efficiency of the compressor pump. It pulls information from the revolutions per minute (RPM) of the motor and the volume of air that the cylinder can displace. This number is more of a theoretical measurement, while you can also measure CFM in terms of delivered air, or how much is actually discharged. This number is called CFM FAD, which stands for free air delivery, and is useful for measuring delivery to certain tools.