Compressed Air Units

Here's a little overview of the most common units used in compressed air land.

New to compressed air? Here are the most common units used in compressed air land, with some extra explanations, to get you started.


Pressure is one of the main units for compressed air. When we compress air, the compressed air pushes back – it wants to decompress. This creates a force on all areas around the compressed air (piping, air receiver, piston, etc). 

This is the pressure – a force per area.  The two most common units are bar and psi. 


Volume is pretty simple and doesn’t need much explanation. In case of compressed air systems, volume is often used to express the size of air receivers.

The most common units are liters, cubic meters and gallons.

Air Flow

Air flow, is volume per time unit. It is how much air flows through a piper per minute for example.

(to be more precise, we should use the word ‘volume flow’ since there’s also a thing called ‘mass flow’.)

If we talk about ‘air flow’ without any other mention of temperature or pressure, we are talking about the actual air flow in the pipe (you will understand what I mean when you read the next point about FAD).

Air flow is commonly expressed as m3/min, l/s or CFM.

Compressor Capacity

The compressor capacity is the amount of air that a compressor produces. The output of a compressor will vary according to the inlet and the outlet conditions of the compressor – inlet temperature, inlet pressure, and out let pressure.

To be able to easily compare air compressors, we use “Free Air Delivery” (FAD), to state the compressor capacity.

FAD = The flow at the compressor outlet calculated back to inlet conditions.

To understand this, just think of a compressor that compressed air at a certain rate. We let the compressed air blow-free to make it expand back to ambient conditions. And THAT is what we ‘measure’ when we talk about FAD.

As you see, the air ‘blown free’ is the same amount of air as the air that the compressor takes in.

To make a fair comparison, we often do this with standardized inlet conditions and measurement method, as described in ISO 1217.

Another way of expressing  this is nl/s (normal liter per second) or SCFM (standard cubic feet per minute). The only difference are the reference conditions.