A good load plan always starts with one simple question: how much space and weight are really available? Brochures talk about a "20-foot" or "40-foot" container, but the numbers you plan with are the inner dimensions, the door opening and the payload of the actual unit you will load.
Outer dimensions, inner dimensions and the door opening
Outer (external) dimensions follow ISO 668 and matter to ships, terminals and roads. For example, a 40-foot container is 12192 mm long outside, and the high cube version is 2896 mm high outside (9 ft 6 in). For cargo, what counts is the inner size, because the walls, floor and roof take up space.
The door opening is always smaller than the inner dimensions. Something that fits inside the container will not necessarily pass through the door. Example: a 20GP is about 2393 mm high inside, but its door is about 2280 mm high. A crate 2350 mm tall will fit inside but cannot be pushed in upright.
Typical figures from the Stowage.id catalogue
| Type | Inner size (L×W×H, mm) | Door (W×H, mm) | Nominal capacity | Tare | Max payload |
|---|---|---|---|---|---|
| 20GP | 5898 × 2352 × 2393 | 2340 × 2280 | ±33.2 m³ | 2230 kg | 28250 kg |
| 40GP | 12032 × 2352 × 2393 | 2340 × 2280 | ±67.7 m³ | 3750 kg | 26730 kg |
| 40HC | 12032 × 2352 × 2698 | 2340 × 2585 | ±76.4 m³ | 3940 kg | 26540 kg |
All three use a maximum gross mass of 30480 kg in this catalogue. Note that the payload of a 40GP and 40HC is actually lower than a 20GP, because a 40-foot box has a heavier tare while the gross limit is the same. This matters for heavy cargo, as the next lesson explains. Full figures are in the container dimensions table.
Nominal CBM vs usable CBM
Nominal capacity is the product of the inner dimensions: geometrically empty space. In practice, the space actually filled with goods is smaller because:
- carton sizes rarely match the container's length, width and height, so gaps remain;
- you need room for hands, forklifts and securing material (dunnage, straps, airbags);
- some goods must not be stacked up to the roof;
- pallets add height and leave gaps at the sides.
In the field, many planners use rough rules of thumb of about 25–28 m³ for a 20GP, 55–58 m³ for a 40GP and 65–68 m³ for a 40HC. These are only starting points; the real result depends on your packaging and must be calculated, not guessed.
Why the CSC plate and door markings decide
Catalogue figures are typical. Each container unit can differ depending on the manufacturer, year of build and series. Containers used in international shipping must carry a CSC safety approval plate (the IMO Convention for Safe Containers). The plate states, among other things, the maximum operating gross mass and the allowable stacking load. The container door usually also shows the tare, maximum gross mass, payload (net) and cubic capacity.
Therefore:
- Use the catalogue for early planning and for booking the container type.
- When the container arrives at the warehouse, check the tare and maximum gross mass on the CSC plate and door markings.
- If the numbers differ, the figures on the physical container apply. Update your plan.
Remember too that weight limits on roads, bridges or warehouse lifting equipment can be lower than what the container is rated for. A container that may carry 28 tonnes does not mean your truck may travel at that weight.
In short: plan with inner dimensions, check the door opening for tall items, and confirm weights on the CSC plate of the actual unit.