An aircraft runs out of space before it runs out of payload, so airlines charge for whichever is greater: the weight of your cargo, or the space it takes up.

The formula

For air freight, volume weight in kilograms is length × width × height in centimetres, divided by 6,000. The chargeable weight is the greater of that figure and the actual gross weight.

A carton of 60 × 50 × 40 cm weighing 12 kg has a volume weight of 20 kg, so you pay for 20.

Five ways to pay for less

1. Right-size the cartons. Empty space inside a box is the most expensive air you will ever ship.

2. Avoid pallets on light cargo. A pallet adds 15 cm of height and its own volume.

3. Stack to the limit. Ask us for the maximum pallet height on your route and build to it.

4. Mix dense and light goods. In a consolidation, heavy items offset bulky ones.

5. Consider ocean. If the volume weight is far above the real weight, the cargo may suit sea freight.