Aluminum is a heavy cargo, and that single fact reorganises every loading decision. A 20-foot container loaded with 2 mm sheet reaches its weight limit while the box is roughly a third full. The empty space is not a planning failure; it is what dense cargo looks like.
Which means the usual intuition — bigger box, better rate per tonne — is wrong often enough to be worth checking every time.
The three boxes, by the numbers
Published figures vary slightly between container owners and vintages, so treat these as the working range rather than a constant:
| Container | Internal volume | Max payload |
|---|---|---|
| 20 ft (20GP) | ~33.1 m³ | ~28,200 kg |
| 40 ft (40GP) | ~67.7 m³ | ~26,700 kg |
| 40 ft high cube (40HQ) | ~76.2 m³ | ~26,600 kg |
Read the payload column again. The 40-foot containers carry less weight than the 20-foot. That is not a typo. Gross weight ratings are similar while tare weight is higher, so the difference is subtracted from the payload. A 40GP has roughly twice the volume of a 20GP and about 1,500 kg less payload.
For a cargo where weight is the binding constraint, that has a direct commercial consequence: if 40-foot freight is more than about 1.4 times the 20-foot rate on your lane, two 20-foot containers move more aluminum for the same money.
The limit that binds is usually not the container
Payload is the number printed on the box. It is rarely the number that applies.
Road limits. In the US, the federal gross vehicle weight limit is 80,000 lb, about 36.3 tonnes, and that covers tractor, chassis, container and cargo together. Subtract the hardware and the cargo allowance for a 40-foot box lands somewhere around 20 tonnes — well under the container's own payload. Similar national limits exist across Europe, and they vary by country and by axle configuration.
So a 40-foot container rated for 26.7 tonnes may only be legally deliverable with 20 tonnes inside it. If your material is going by rail or road beyond the port, the road limit is the one to design the load against, not the container rating.
Handling limits. A bundle over about 5 tonnes needs equipment that not every warehouse has. A single coil over 10 tonnes needs a crane, not a forklift. Loading is possible; unloading at your end may not be.
Floor loading. Point loads from a heavy bundle on a small footprint can exceed the container floor rating. Spreaders or timber bearers solve it, and they take up height.
Working out a real load
Take 5052 sheet, 2.0 mm × 1000 × 2000 mm.
Theoretical weight per sheet: 2.0 × 1000 × 2000 mm = 4,000,000 mm³, or 4,000 cm³, at 2.68 g/cm³ = 10.72 kg.
A practical bundle is around 3 tonnes, so about 280 sheets, stacked roughly 560 mm high plus bearers.
On the floor of a 20-foot container — internal footprint around 5.90 × 2.35 m — two 1000 mm widths fit across, leaving about 350 mm of side space, and two 2000 mm lengths fit along, leaving about 1.9 m at the door end. That is four bundles on the floor. Stacked two high, eight bundles. At 3 tonnes each, 24 tonnes.
Push to three high and you are at 36 tonnes of bundle against a 28.2-tonne payload. Weight limit reached, volume barely used. Nine bundles at 3 tonnes gives 27 tonnes and consumes perhaps 10 m³ of the 33 available.
Now run the same calculation on 10 mm plate, 1500 × 3000 mm: about 120 kg per plate, so a 2.5-tonne bundle is roughly 20 plates and only 200 mm thick. Different geometry, same conclusion — the scale reads the limit first.
What actually changes the number
Bundle weight. Heavier bundles mean fewer pieces to handle and less dunnage, but they need lifting equipment at both ends. Bundles in the 2–3 tonne range are the usual compromise.
Dunnage. Timber bearers, saddles and chocks all consume height and volume. On a 20-foot load of dense sheet this is irrelevant; on a 40HQ full of extrusion it is not.
Coil orientation. Coils shipped eye-to-sky use floor space proportional to their diameter and can be stacked less efficiently than sheet bundles. Coils on saddles use more floor and less height. For aluminum coil the choice is driven by deformation risk, not by the loading diagram.
Mixed sizes. A container with four sizes in it will not tile perfectly. Expect 5–10% of the theoretical capacity to be lost to gaps, and price the freight per tonne accordingly rather than assuming a full box.
The loading plan that is worth writing
A loading plan is not a diagram for its own sake. It fixes three things that later become arguments.
Bundle positions and weights. Which bundle goes where, and what it weighs. This is what lets you check a partial delivery, and what an insurer asks for after damage.
Center of gravity. Heavy bundles centered and low. A load that is heavy at the door end shifts under braking, and the shift damages the front face of everything behind it.
Restraint method. Timber blocking against the side walls, straps to the floor rings, or both. Aluminum is slippery against aluminum — a stack with nothing between the layers and nothing against the walls will move.
Ask for the loading plan as a document before loading, not as photographs after. The photographs confirm the plan was followed; they cannot substitute for one.
Coil is the exception that goes the other way
Sheet and plate hit the weight limit with volume to spare. Coil can do the opposite, and the reason is geometry.
A coil standing eye-to-sky occupies floor space equal to its diameter times its width, while most of that footprint is the hole in the middle. A 1,500 mm diameter coil of 1,200 mm width uses 1.8 m² of floor for a coil that might weigh 5 tonnes. Run that across a 20-foot floor: the box is 2.35 m wide, so one coil across, and 5.9 m long, so three coils along. Three coils, 15 tonnes, and the container is full at half its payload.
Smaller diameter coils pack better. At 1,200 mm diameter and 1,000 mm width, four fit along the length and two across the width — eight coils, and at 3 tonnes each that is 24 tonnes, close to the limit.
So the loading question for coil is not "how many tonnes" but "what diameter and what weight", and the two are linked by the gauge and width. A heavy coil of thin wide material is a large coil; a heavy coil of thick narrow material is a small one.
Two rules follow.
Never assume a coil container is full by weight. Check the coil schedule — diameters, widths, weights — against the internal dimensions before booking. This is where a "27 tonnes" quote turns into a 15-tonne shipment and a freight cost per tonne nobody planned for.
Orientation is a quality decision, not a space decision. Eye-to-sky with a matched saddle and radial strapping prevents telescoping, which is what destroys a coil for automatic feeding. Laying coils on their side saves height and risks the bore. Take the space penalty.
Choosing the box
| Situation | Box | Reason |
|---|---|---|
| Sheet or plate, over 20 t | 20 ft, or two | Payload per box is higher and the volume is wasted anyway |
| Coil, 15–20 t | 20 ft | Weight-limited; 40 ft adds cost, not capacity |
| Extrusion, low density | 40HQ | Volume-limited, and the extra height is usable |
| Mixed order, destination near port | 40GP | Fewer boxes to handle, no road limit issue |
| Inland delivery in the US or EU | Check the road limit first | It may be well below the container rating |
The pattern: dense cargo goes in 20-foot boxes, bulky cargo goes in 40HQ, and the road limit overrides both when the destination is inland.
One question worth asking early
Before a quote is finalised, ask what the load plan weighs. A supplier who quotes 27 tonnes into a 40-foot box either knows your destination allows it or has not checked. Either answer is useful, and both are cheaper to discover now than at a weigh station.
If you want a load plan checked against the destination's road limits before booking, send us the order weights and the delivery address region. The container calculation is quick; the road limit is the part that needs looking up.