Free air delivery, or FAD, is the volume of air a compressor actually delivers, measured at the inlet conditions and normalised to a reference state. It is the only number that tells you the true output, and it is always lower than the theoretical displacement.
FAD vs displacement
Displacement is the swept volume of the airend, a theoretical figure that ignores leakage, losses and inlet restrictions. FAD is the measured delivered air, so it is the figure to use for sizing. Comparing machines on displacement rather than FAD consistently overestimates output.
Why the measurement conditions matter
FAD depends on inlet temperature, pressure and humidity, so different manufacturers must quote it at the same reference conditions for a fair comparison. Always check the reference standard, usually ISO 1217, when comparing datasheets.
Using FAD when buying
Match the compressor FAD to your measured air demand at the working pressure, then add a growth margin. Relying on displacement or horsepower leads to an under-sized or over-sized machine, either of which costs money in the long run.