Volume is a measurement. Chargeable weight is a rule. Most of the confusion about a freight quote comes from mixing them up, so here is the arithmetic, with numbers you can check yourself.
Step 1: the volume
For one carton size:
CBM per carton = length x width x height (in metres)
Line CBM = CBM per carton x cartons
Shipment CBM = sum of the lines
In centimetres, divide the product by 1,000,000. Keep one line per carton size. Averaging the sizes of different cartons gives a wrong total, because volume does not average linearly.
Step 2: the billing rule
The same volume becomes different numbers depending on the service.
Air and express compare gross weight with volumetric weight:
Volumetric kg = total cm3 / divisor
Planning chargeable kg = max(gross kg, volumetric kg)
A divisor of 5,000 corresponds to 200 kg per cubic metre. A divisor of 6,000 corresponds to about 166.67 kg per cubic metre. Neither applies to every service or route, so use the one on your quotation.
Sea LCL usually bills on W/M: cubic metres compared with gross tonnes, whichever is greater.
A worked example
Take a shipment of 0.72 m3 (720,000 cm3) weighing 120 kg gross.
| Basis | Calculation | Result |
|---|---|---|
| Express, divisor 5,000 | 720,000 / 5,000 = 144 kg, against 120 kg gross | 144 kg |
| Air, divisor 6,000 | 720,000 / 6,000 = 120 kg, against 120 kg gross | 120 kg |
| Sea LCL, W/M | 0.72 m3 against 0.12 tonnes | 0.72 W/M |
Same cartons, three different billing numbers. The goods did not change, only the rule did.
The part that surprises people: rounding
Two cartons, each 51 x 40 x 30 cm and 10 kg. At a divisor of 5,000, each one has 12.24 kg of volumetric weight, so the shipment compares 24.48 kg volumetric with 20 kg gross.
- Round the whole shipment up to the next 0.5 kg: 24.5 kg
- Round each carton up to the next 0.5 kg, then add: 25 kg
Neither is wrong. Which one applies depends on your tariff, so it is worth asking. With mixed carton sizes, adding each carton's higher weight can also differ from taking the higher of the shipment totals.
Check before you book
- Measure the packed outer carton, not the product.
- Ask for the divisor, the rounding rule and the minimum charge.
- Check whether the service bills per piece.
These figures are for planning, not a carrier tariff.
Disclosure
I built Livnaro, a free browser CBM worksheet that does this arithmetic for mixed carton sizes and shows the formula with each result. The two-carton example is also on its chargeable weight calculator, and the measuring details are in how to calculate CBM. This post was drafted by an AI assistant from those pages.
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