Shipping Container Types and Sizes: Dimensions, Capacity and Payload

External name, internal reality

A container is named for its external length, but nothing you load is affected by the outside. Corrugations, corner castings, flooring and door frames all take space, so the usable interior is meaningfully smaller than the name suggests — and the door aperture is smaller again than the internal cross-section.

Two limits govern every load and they run out at different times. Volume is bounded by internal dimensions. Weight is bounded by payload, which is the container's maximum gross weight minus its own tare — both stencilled on the door, and both varying between individual boxes of the same nominal size.

Dense cargo hits payload long before it fills the space. Light cargo fills the space long before it approaches payload. Knowing which of the two constrains your shipment determines whether packaging redesign or different equipment is the answer, and it is the first thing to work out.

Why a 40ft does not carry twice a 20ft

This surprises people every time. A 40ft container gives roughly twice the volume of a 20ft, but its payload is often lower — because maximum gross weight is set by structural rating and by road and lifting limits rather than by length, while the 40ft's tare is nearly double.

The consequence drives most equipment decisions in ocean freight. Heavy cargo — tiles, stone, machinery, liquids in drums, cabling — moves in 20ft units, often several of them, because a 40ft would be paid for and half empty. Light cargo — furniture, packaging, textiles, plastics — moves in 40ft high cubes, where the extra foot of height adds roughly nine cubic metres at little freight premium.

The rough dividing line is cargo density. Above about 850 kg per cubic metre a shipment will hit the payload of a 20ft before filling it. Below about 400 kg per cubic metre it will fill a 40ft high cube before approaching the weight limit.

Specialised equipment

Beyond dry boxes, a range of equipment exists for cargo that cannot travel in a standard container. Each trades capacity or convenience for a specific capability, and each has its own loading rules and availability constraints.

• Refrigerated (reefer) — temperature-controlled, with the machinery occupying internal length and the T-floor and air return needing to stay clear. Capacity is materially lower than a dry box of the same nominal size, and free time allowances are typically shorter
• Insulated or ventilated — passive temperature protection or airflow without active refrigeration, used for coffee, cocoa and similar commodities
• Open top — removable tarpaulin roof for cargo loaded by crane from above, or that exceeds internal height. Cargo protruding above the roof line attracts out-of-gauge surcharges
• Flat rack — collapsible or fixed end walls with no sides or roof, for machinery, vehicles and oversized cargo. Loading and securing are specialist work
• Platform — a flat base with no walls at all, for very heavy or very wide cargo
• Tank container — an ISO frame around a cylindrical tank, for bulk liquids, gases and powders. Cleaning certificates and previous-cargo history are part of the booking
• Half height — reduced height for dense bulk cargo such as ore and stone, where payload is exhausted at low volume
• Hard top — a removable steel roof, combining weather protection with top loading

Reading the markings on the door

Everything you need to plan a load is stencilled on the container itself, and reading it takes seconds. Beside the eleven-character identifier sits a four-character ISO size and type code: the first character is length, the second is height and width, and the last two describe the type.

Below that are the weights: maximum gross weight, tare weight and net or payload capacity, usually in both kilograms and pounds. Take these from the actual container rather than a table — tares vary by manufacturer, by age and by any repairs, and a several-hundred-kilogram assumption error propagates straight into your VGM.

Also look for the CSC safety approval plate, which evidences that the container has been examined and is approved for carriage. A container without a valid plate should not be loaded.

What you can actually load

Nominal capacity assumes a perfect rectangular fill that no real load achieves. Plan on 80 to 85 per cent of nominal for floor-loaded cartons, and less again for palletised cargo where the pallet consumes height on every layer.

The space goes to predictable places: the door aperture is roughly 100 mm narrower and 110 mm lower than the internal cross-section, so the last tier often cannot be loaded to full height; corrugated walls take a few centimetres at intervals; cartons packed tight bulge beyond their printed dimensions; dunnage, airbags and edge protection occupy real volume; and cargo cannot be stacked above its own crush rating.

High cubes and the road leg

A high cube's extra 300 mm of internal height is free volume at sea and a constraint on land. Combined vehicle height limits, low bridges, tunnels and some national road regulations restrict where a high cube can travel, and in a few markets they require specific chassis or permits.

This is worth confirming before booking rather than after. Discovering at destination that a high cube cannot reach the delivery address means transloading — unstuffing into a standard container or a truck — at a cost that comfortably exceeds whatever the extra volume saved.