Cubing out, weighing out, and the mistake that costs the most
Every consignment hits one of two limits first. Which one it is determines how many boxes you buy, and a surprising number of shippers find out afterwards.
A container has two capacities and you only get to use one of them. Either you fill the space before you reach the payload limit, or you reach the payload limit with space to spare.
The two numbers
A 40ft high cube holds 76.3 m³ and 26,580 kg. Divide one by the other: 348 kg per cubic metre. Anything denser than that weighs out. Anything lighter cubes out.
Grain is around 750 kg/m³. It weighs out at roughly 46% of the cube — the box is more than half empty and legally full. Packaging foam is around 30 kg/m³. It cubes out at under 9% of the payload, and the axle limit is irrelevant.
Why this decides the equipment
For dense freight, the 20ft unit is the right answer and it is counter-intuitive. A twenty carries more payload than a forty — 28,250 kg against 26,730 — because the box itself weighs less and the maximum gross is the same 30,480 kg for both. Half the space, more weight allowance.
Shippers who have only ever bought by the box find this hard to believe, and it is worth ten to fifteen per cent on a dense export programme.
The awkward middle
Mixed consignments are where it gets genuinely difficult, because the density that matters is the density of the whole load, and it changes every time the order mix changes. That is what the load planner on the equipment page is for: put the actual handling units in, and it will stow them and tell you which limit you hit.