The single most common billing surprise
A shipper books 40 kilograms of cushions to London, is quoted a per-kilo rate, and receives an invoice for 120 kilograms. Nothing has gone wrong and nobody has overcharged them. The consignment was billed on chargeable weight, which is what carriers actually use, and the cushions took up the space of 120 kilograms even though they weighed 40.
Air and courier freight is sold by the space a consignment occupies as much as by its mass, because an aircraft runs out of volume long before it runs out of payload on most lanes. Understanding the calculation before you book is the difference between a predictable cost and an argument after the fact.
How the calculation works
Chargeable weight is the greater of two numbers: the actual gross weight of the consignment, and its volumetric weight. Volumetric weight is derived from the dimensions by multiplying length, width, and height in centimetres, then dividing by a fixed divisor.
The divisor is the part that differs by mode, and it is where most confusion starts. Air freight uses 6000. Express and courier operators typically use 5000, which produces a higher chargeable weight for the same box. Sea freight does not use a divisor at all: it is priced per cubic metre, so mass barely matters until a container approaches its payload limit.
- Air freight: (L × W × H in cm) ÷ 6000 = volumetric kg
- Courier / express: (L × W × H in cm) ÷ 5000 = volumetric kg
- Sea freight: (L × W × H in cm) ÷ 1,000,000 = cubic metres, billed per m³
- Chargeable weight = whichever is greater, actual or volumetric
A worked example
Take two cartons, each 100 cm long, 80 cm wide, and 60 cm high, with a combined actual weight of 90 kilograms. The volume is 100 × 80 × 60 = 480,000 cubic centimetres per carton, so 960,000 for the pair.
By air, 960,000 ÷ 6000 gives 160 kilograms volumetric. That is greater than the 90 kilograms actual, so the consignment is billed as 160 kilograms. By courier, the same cartons divided by 5000 give 192 kilograms — the identical shipment costs more by courier purely because of the divisor. By sea, the same cartons are 0.96 cubic metres and the weight is close to irrelevant.
What you can actually do about it
When volumetric weight governs, reducing mass saves nothing at all — only reducing volume does. That reframes the packing problem entirely.
- Right-size the carton. Air inside the box is billed at the same rate as the goods.
- Avoid oversized outer cartons used purely for protection; use tighter packing plus better internal cushioning.
- Nest and disassemble where the goods allow it.
- Stack to a standard pallet footprint rather than an irregular one, so no space is wasted.
- Ask which mode the crossover favours: bulky, low-value cargo is usually far cheaper by sea.
- Give your forwarder real dimensions at quotation stage, not estimates — a quote against wrong dimensions is not a quote.
Check your own numbers
Our quote calculator applies the correct divisor per mode and shows both the actual and volumetric figures side by side, so you can see which one governs before you commit to a booking. Switching mode on the same cargo also makes the air-versus-sea crossover obvious in your own numbers rather than in the abstract.