Container loading 101: fitting more into every box

Internal dimensions and payload, by container type

A standard 20ft dry container has an internal length of roughly 5.9 m, a width of about 2.35 m and a height of about 2.39 m, giving around 33 cubic metres of capacity, with a maximum payload typically in the 28,000 to 28,200 kg range depending on the tare weight of the specific unit. A standard 40ft dry container roughly doubles the length to about 12.03 m at the same width and height, giving around 67 cubic metres, but the payload only rises to around 26,500 to 28,600 kg, because the heavier steel structure of a longer box eats into the weight allowance even as volume grows. This is why dense cargo often ships better in 20ft units and bulky, light cargo in 40ft units: the payload-per-cubic-metre ratio is very different between the two.

A 40ft high cube adds about 30 cm of internal height over a standard 40ft, taking it to roughly 2.69 m, for around 76 cubic metres at a similar payload to the standard 40ft. That extra height is pure upside for cargo that is volume-bound rather than weight-bound: a producer shipping foam produce boxes or empty pallets gains real capacity from the high cube, while a producer shipping bagged rice gains nothing, because rice hits the weight ceiling long before it reaches the roof.

A 40ft reefer is the exception to "same footprint, more or less space." Its internal length runs to around 11.6 m and internal height to around 2.5 m, both noticeably less than a 40ft dry box, because the insulated walls, the refrigeration unit at the front of the container and the raised T-bar floor all consume space that a dry container does not give up. Reefer payload is typically lower too, often in the 27,000 to 29,000 kg range before accounting for the load line restriction described below. Anyone converting a dry-container carton count to a reefer count needs to redo the calculation on the reefer's actual internal dimensions, not the dry container's.

How pallet footprint drives the carton count

Once cargo is palletised, the pallet footprint, not the carton size, is what determines how much of the container floor is actually usable. The two footprints in common use are the Euro pallet at 1200 x 1000 mm and the US GMA pallet at 1219 x 1016 mm (48 x 40 inches). Neither one divides evenly into a 20ft or 40ft container width, which is where fit is won or lost.

In a 20ft container with roughly 2.35 m of internal width, Euro pallets loaded with the 1000 mm side against the wall fit two across with a small gap left over; loaded the other way, with the 1200 mm side against the wall, only one row fits per that orientation before the second pallet overhangs the usable width. Down the 5.9 m length, two-across Euro pallets in the narrower orientation typically stack 11 pallet positions deep, for around 20 to 22 standard pallet positions in a 20ft box depending on exact loading pattern and door clearance. GMA pallets, being slightly larger, generally fit one fewer position across the same length.

In a 40ft container, the same width constraint applies, but the extra length roughly doubles the pallet count to around 23 to 24 Euro pallet positions, or a comparable slightly lower count for GMA pallets. The practical lesson is that pallet orientation, not just pallet count, changes the answer: rotating pallets 90 degrees can gain or lose one full row along the length of the container, and that single row is often the difference between needing an extra container and not.

Floor loading versus palletised, and when each wins

Floor loading, stacking cartons or sacks directly on the container floor without pallets, recovers the volume that pallets themselves occupy: a pallet is typically 12 to 15 cm tall and takes up floor area that carries no product. For cargo that is genuinely volume-bound and does not need to move quickly through a warehouse at destination, floor loading commonly recovers 10 to 15 percent more usable volume than the same cartons palletised, because there is no pallet deck, no gap between pallet rows and no wasted height under the beams.

Palletising wins whenever handling speed and labour cost at either end outweigh the volume lost to the pallet itself. A palletised container can be unloaded with a forklift in well under an hour; a floor-loaded container of the same cargo often needs several workers hand-stacking cartons for three to four hours or more. Palletising also protects fragile or stacked cartons from crushing, since pallets distribute weight evenly and stop cartons on the floor from bearing the full weight of everything stacked above them. As a rule of thumb: floor-load dense, robust, uniform cargo moving to a buyer with manual unloading capacity; palletise anything fragile, mixed-SKU, or moving to a distribution centre that unloads by forklift.

Weight distribution and axle limits

A container can sit comfortably under its payload plate and still be an illegal load once it is on a chassis, because the container payload limit and the road axle limit are two entirely separate ceilings. Road authorities regulate weight per axle group on the truck and trailer, not just the gross weight of the container, and a container loaded heavy at one end concentrates weight over whichever axle sits underneath that end. A shipment that is well within the container's rated payload can still overload the drive axle or the trailer tandem if all the heavy pallets were loaded toward the doors or toward the nose.

The practical fix is to spread weight evenly along the length of the container and, where cargo is genuinely uneven (some pallets much heavier than others), to alternate heavy and light pallets rather than grouping all the heavy ones together. Forwarders and truckers will generally flag an axle problem before a container leaves the yard, but the cost of a re-stow discovered at the scale house, or a fine for an overweight axle on the highway, is entirely avoidable by loading with distribution in mind from the start, not just total weight.

Reefer airflow, the T-bar floor and the load line

A reefer container cools by pushing air from the refrigeration unit at the front, down through the T-bar floor channels, up through and around the cargo, and back to the unit as return air, in a continuous loop. The T-bar floor is not just a walking surface, it is the supply-air duct: cartons or bags placed directly against the floor without gaps block that duct and starve the load of cold air exactly where it is needed most.

The red load line marked on the interior walls of a reefer shows the maximum stack height for that unit. Cargo stacked above the load line blocks the return-air path across the top of the load, so air recirculates through the top layer of cartons instead of circulating down through the whole stack, and the top of the load can run several degrees warmer than the setpoint for the entire transit. Blocking either end of that airflow loop, floor or ceiling, is the most common single cause of a reefer claim: the product itself was fine at loading, the container's compressor worked the whole voyage, and the load still arrived out of temperature because the air simply never reached the middle of the stack.

Cargo also needs to be loaded with a small gap from the side walls and rear doors, not packed flush against them, so air can return along the walls to the refrigeration unit rather than dead-ending against a solid wall of cartons.

Dunnage, bracing and void fill

Loose cargo shifts in transit, and a container that arrives with pallets leaning or cartons collapsed usually lost that fight to an unbraced void, not to rough handling. Dunnage bags (inflatable air bags placed between the last row of pallets and the container doors) are the standard fix for the final gap, since a container length rarely divides evenly into whole pallet positions and the last row is almost always short of a full stack.

Bracing boards or straps across pallet rows stop lateral movement during the sharp accelerations and vessel roll a container experiences at sea, which are far harder on a load than anything it sees on a truck. Void fill, whether reused pallets, air pillows or honeycomb board, matters most in mixed-height loads where a short pallet leaves an open pocket above it that a taller pallet in the next row can tip into. None of this is expensive relative to the cargo value, and skipping it is the most common reason an otherwise well-planned, correctly weighted container still arrives with damage.

The pre-load checklist for a reefer

A reefer load starts before the first pallet goes in. A pre-trip inspection (often abbreviated PTI) runs the unit through its own diagnostic cycle to confirm the compressor, fans, defrost cycle and controller are all working before the empty container reaches the loading dock; skipping this step means the first sign of a fault is a temperature excursion at sea, when nothing can be done about it. The container also needs pre-cooling to the setpoint temperature before loading begins, not after the doors close, since loading warm product into a warm box means the unit is fighting to pull down both the cargo and the container structure at once, and it may not fully catch up for the rest of the voyage.

At loading, check that the T-bar floor is clear and undamaged, that cargo is stacked below the red load line, that there is a clear air gap at the walls and rear doors, and that the setpoint, and where relevant the humidity and ventilation settings, are confirmed and locked before the doors are sealed. A final check of the door seal and the working condition of the temperature recorder or data logger closes the loop, since a dispute over whether a load left origin at the right temperature is far easier to resolve with a logger record than without one.