HP, CFM, AND PSI
The horsepower rating of a compressor’s motor tends to be used as a rule of thumb for determining the size of unit you need, and a quick bit of maths — based on the rough estimate that a proper single-stage compressor, per true hp input, will deliver about 3– 4cfm at 90psi — will tell you where the compressor you’re looking at lands.
A cubic-feet-per-minute (cfm) rating tells you the volume of air that a compressor produces and is an important consideration, as most tools require a certain cfm at a particular air pressure to function. Some units may state a displacement cfm rating that appears higher; this is calculated based on the size and speed of the compressor pump but as measured at zero pressure, so it fails to take into account inefficiencies and isn’t generally accurate about what the tool may require. So it’s key to know what cfm the compressor is capable of at 90psi, especially on smaller compressors, as this is the pressure required to run most tools.
That air pressure is almost always rated in pounds per square inch (psi) and while most air tools require 90psi to function properly, maintaining that 90psi at the tool is just as important. To achieve this, a higher shut-off pressure is required. Most hobby and smaller commercial compressors are a single-stage design that shuts off when pressure has built to roughly 125–135psi, kicking back in when pressure drops to 100psi or below, whereas larger industrial compressors are two-stage, where pressure builds to 90psi at the first stage and 175psi at the second stage.
EXPERT OPINION
“You need to think about what pressure is required to operate your tools, and how much flow is needed to maintain the tools’ performance. Most workshop tools will operate at 90psi, so, with a given compressor output pressure of around 110psi, the only real decision is how much air you need (cfm). This is where the horsepower of the compressor comes in, as the flow is directly related to the power being put into the pump. More power gives a greater flow,” says Richard