A compressor listing usually leads with a cubic feet per minute (cfm) figure printed in large type near the price. On piston compressors, that number is very often displacement, the volume swept by the piston per minute, calculated from the bore, stroke and running speed before any air is actually lost to valve timing, heat or leakage past the piston rings.

Free air delivery (FAD) is the figure that tells you what actually leaves the compressor and reaches your airline, measured at atmospheric pressure and temperature, at a stated working pressure. It is always the lower of the two numbers, because every real compressor loses some air to inefficiency between the swept volume and the outlet. The gap between displacement and free air delivery is not a flaw in the machine; it is a property of how the two figures are calculated, and a listing that only states one of them is not telling you which.

Why the difference matters when you size a compressor

If you work out how much air your tools need and then buy a compressor rated to match that number, the rating has to be free air delivery. A machine bought against its displacement figure will fall short in practice, sometimes by a wide enough margin that a DA sander starts to lose pressure mid-cut or a spray gun stops giving even coverage. Sizing against the wrong figure is the single most common reason a compressor that looked adequate on paper turns out not to be.

Working out what your own tools actually need is covered separately, along with a worked example of the ratio between the two figures on real listings. Once you have a delivery figure from one listing and a displacement figure from another, comparing them means converting both to the same unit first. The compressed air unit converter handles the arithmetic between litres per minute, cubic feet per minute and the other units listings use, though it only converts a figure between units; it cannot tell you whether the figure itself is displacement or free air delivery, or turn one into the other.

Where this leaves you as a buyer

Neither figure is wrong. Displacement describes the machine; free air delivery describes what it does for you. The trouble is that most listings put the bigger, more flattering number where you will see it first, and leave the smaller one, if it appears at all, in a specification table further down the page. Reading a listing well means finding both, converting them to the same unit if they are not already stated in one, and knowing which one to act on.

A compressor listing usually leads with a cubic feet per minute (cfm) figure printed in large type near the price. On piston compressors, that number is very often displacement, the volume swept by the piston per minute, calculated from the bore, stroke and running speed before any air is actually lost to valve timing, heat or leakage past the piston rings.

Free air delivery (FAD) is the figure that tells you what actually leaves the compressor and reaches your airline, measured at atmospheric pressure and temperature, at a stated working pressure. It is always the lower of the two numbers, because every real compressor loses some air to inefficiency between the swept volume and the outlet. The gap between displacement and free air delivery is not a flaw in the machine; it is a property of how the two figures are calculated, and a listing that only states one of them is not telling you which.

Why the difference matters when you size a compressor

If you work out how much air your tools need and then buy a compressor rated to match that number, the rating has to be free air delivery. A machine bought against its displacement figure will fall short in practice, sometimes by a wide enough margin that a DA sander starts to lose pressure mid-cut or a spray gun stops giving even coverage. Sizing against the wrong figure is the single most common reason a compressor that looked adequate on paper turns out not to be.

Working out what your own tools actually need is covered separately, along with a worked example of the ratio between the two figures on real listings.

Where this leaves you as a buyer

Neither figure is wrong. Displacement describes the machine; free air delivery describes what it does for you. The trouble is that most listings put the bigger, more flattering number where you will see it first, and leave the smaller one, if it appears at all, in a specification table further down the page. Reading a listing well means finding both and knowing which one to act on.

Some listings compound the problem by mixing units: one figure in cfm, the other in l/min, pressure given in bar on one line and psi on the next. Before you work out the ratio between displacement and free air delivery, put both figures on the same units with the Compressed Air Unit Converter, which shows the conversion factor used for flow and pressure so you can see exactly how one number was turned into the other.

Signs the figure on the page is unlabelled

• A single cfm or l/min number with no word nearby telling you whether it is displacement or free air delivery.
• A flow figure with no pressure stated next to it, since the same compressor delivers less air at a higher working pressure and a figure without a pressure is not comparable to anything.
• Round, generous-looking numbers positioned above the price, with a smaller and less rounded figure lower down the specification table.
• A noise or power figure quoted with confidence while the flow figure carries no label at all, suggesting the gap was not an oversight.