The unit of measurement used in air shipments is the so-called "Volumetric Weight" or "Chargeable Weight," also abbreviated as "KG ACW."

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Although it uses the widely recognised term "KG," this can be misleading for the average person. In reality, it’s a specific concept within the airline industry (although it has also been adopted by couriers in recent years).

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To explain, let’s imagine we have a very light shipment of 1 cubic metre, alongside another, hypothetical, much heavier shipment of 0.1 cubic metres.

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For an airline, 1 cubic metre of light material (such as used personal effects) is equivalent to 167 KG ACW, even if this cubic metre only actually weighs 1 KG. The airline will still charge for 167 KG ACW. Therefore, for light shipments, the main factor driving up costs is the volume.

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However, if the shipment exceeds the notorious 167 KG limit of actual weight (for example, a shipment of bricks or slabs of wood), the chargeable weight is calculated as the actual weight.

So, for instance, in the case of a hypothetical shipment of 0.1 cubic metres with an actual weight of 300 KG, the chargeable weight for the airline is 300 KG ACW.

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For heavy shipments, then, the main cost factor is the actual weight.