Freight answersContainer
Why Is Heavy Cargo Shipped in 20-ft Containers?
Short answer
Because a 20-ft container carries nearly as much weight as a 40-ft one in half the space. On one carrier's specs the 20-ft payload is about 28,200 kg against about 28,800 kg for a 40-ft, while the 40 has about twice the volume. Dense cargo reaches the weight limit long before it fills the box, so a 40 would carry little extra weight for the extra cost; the 20 is the better fit.
What it depends on
- Cargo density: how many kilograms each cubic meter of packed cargo weighs.
- The payload ratings of the actual boxes available.
- Freight rates for a 20 versus a 40 on the lane.
- Road limits on the drayage leg at each end.
Weighing out vs. cubing out
Transport geographers describe the two ways a container fills up: it either "weighs out," reaching its weight limit, or "cubes out," running out of space. For heavy goods such as copy paper, the 20-footer is the optimum unit: about 1,700 units weighing just over 20,000 kg fill its permissible weight, while a 40-footer carries only about 27.6% more.[2]
| Container | Volume | Max payload | Payload per m³ of space |
|---|---|---|---|
| 20-ft standard | 33 m³ | 28,200 kg | about 855 kg |
| 40-ft standard | 67 m³ | 28,800 kg | about 430 kg |
Computed from Maersk specifications.[1] Payload values vary by box; the same carrier's FAQ lists slightly different figures.[3]
Read the last column as a rule of thumb: cargo that packs heavier than roughly 430 kg per cubic meter of container space will weigh out a 40 before filling it, and much of it fits a 20 better. Because boxes are never packed 100% full, the real crossover is lower still.
Example
A tile exporter's pallets pack at about 1,000 kg per cubic meter. A 20-ft box reaches its payload with roughly 28 m³ loaded; a 40-ft box would hit the same weight limit while more than half empty. Two 20s carry twice the tiles of one 40.
Limitations
This compares container ratings only. Road weight limits at origin and destination can cap the load lower than either rating, and the cheapest choice depends on the rates offered for each box size.
Related
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Related tools
Sources
- Maersk, “Our fleet: equipment specifications, dry containers (PDF),” Carrier equipment specification. Accessed September 30, 2026.
- J.-P. Rodrigue, Hofstra University, “Container Weighting and Cubing Out (The Geography of Transport Systems),” Academic reference. Accessed September 30, 2026.
- Maersk, “What is the cargo weight limit for a container?,” Carrier FAQ. Accessed September 30, 2026.
General operational information, not legal, customs or regulatory advice. Rules and rates change; confirm current requirements with the agency, carrier or contract that governs your shipment.