ISO standards for shipping containers are the set of rules that fix their dimensions, strength, markings and security seals, so that any container can be loaded onto any ship, train or truck in the world without adaptation. The four you need to know are ISO 668 (dimensions and ratings), ISO 1496 (specification and testing), ISO 6346 (coding and identification) and ISO 17712 (mechanical security seals). Alongside them sits the IMO's CSC Convention, which is not an ISO standard but governs structural safety through the CSC plate.

Which ISO standards apply to containers?

Container standardization is the reason ocean trade works the way it does. Each standard covers a different aspect, and the four complement each other:

Standard	What it governs	Why it matters
ISO 668	Classification, external dimensions and maximum gross mass of series 1 containers	Guarantees a container fits the ship's cell guides, the chassis and the rail car
ISO 1496	Specification and testing by type: general cargo, thermal, tank, platform	Defines the stacking, lifting, rigidity and weatherproofness tests it must pass
ISO 6346	Coding, identification and marking	What makes it possible to track a specific unit at any terminal in the world
ISO 17712	Mechanical security seals for freight containers	Classifies seals and requires tamper evidence
CSC Convention (IMO)	Structural safety and inspection regime	Without a valid CSC plate, a container cannot be used in international transport

The ISO 1496 series is divided by unit type: part 1 covers general cargo containers (closed, ventilated and open top), part 2 thermal containers, part 3 tank containers and part 5 platforms and platform-based containers.

ISO 668: dimensions and maximum gross mass

ISO 668 defines the external dimensions and the designation of each type. The width is always 2.438 m (8 ft); what changes is the length and height. Standard height is 2.591 m (8 ft 6 in) and High Cube height is 2.896 m (9 ft 6 in).

ISO designation	Common name	External length	External height
1CC	20 ft standard	6.058 m (20 ft)	2.591 m (8 ft 6 in)
1CCC	20 ft High Cube	6.058 m (20 ft)	2.896 m (9 ft 6 in)
1AA	40 ft standard	12.192 m (40 ft)	2.591 m (8 ft 6 in)
1AAA	40 ft High Cube	12.192 m (40 ft)	2.896 m (9 ft 6 in)

There is a nuance about maximum gross mass worth understanding. A 2016 amendment to ISO 668 raised the regulatory ceiling to 36,000 kg (79,366 lb) for standard containers of every size except the 10 ft. However, the vast majority of units in circulation are still built and marked with a maximum gross mass of 30,480 kg (67,200 lb), and that is the value that governs your operation. The figure that rules is not the one in the standard but the one stamped on your container's CSC plate.

Dimensions and capacities by container type

These are the reference figures most used in day-to-day operations. Internal dimensions and tare vary slightly between manufacturers, especially on reefers and flat racks, so always check the plate of the specific unit.

Type	Approx. interior (L x W x H)	Approx. volume	Approx. tare
20 ft Dry Van	5.90 x 2.35 x 2.39 m	33 m³ (1,165 cu ft)	2,200 to 2,300 kg
40 ft Dry Van	12.03 x 2.35 x 2.39 m	68 m³ (2,400 cu ft)	3,700 to 3,800 kg
40 ft High Cube	12.03 x 2.35 x 2.70 m	76 m³ (2,684 cu ft)	3,900 to 4,000 kg
20 ft Reefer	5.45 x 2.29 x 2.25 m	28 m³ (989 cu ft)	2,900 to 3,100 kg
40 ft Reefer High Cube	11.58 x 2.29 x 2.50 m	64 to 67 m³ (2,260 to 2,366 cu ft)	3,900 to 4,600 kg
20 ft Open Top	5.90 x 2.35 x 2.35 m	32 m³ (1,130 cu ft)	2,200 to 2,400 kg
20 ft Flat Rack	5.70 x 2.20 m platform	Not applicable	2,500 to 2,900 kg

The internal volume of a reefer is smaller than that of a dry container of the same size because the thermal insulation and the cooling unit take up space. If you work with cold chain, see our line of refrigerated containers.

Standardized container types

Dry Van. Closed general cargo container. The most widely used and the one that defines the standard.
High Cube. Same as the Dry Van but 30 cm (1 ft) taller. Ideal for bulky, light cargo.
Reefer. Thermal container with its own refrigeration unit, covered by ISO 1496-2. Requires an electrical connection on the ship, in the yard and on the truck.
Open Top. No rigid roof; closed with a tarpaulin and removable bows. Allows vertical loading by crane for machinery or parts that do not fit through the door.
Flat Rack. Platform with fixed or collapsible end walls, no sides or roof. Used for oversized cargo, vehicles and structures.
Platform. Just the base, with no walls or end frames. The extreme version of the flat rack, governed by ISO 1496-5.
Tank. A tank mounted inside a frame with standard ISO dimensions, for liquids, gases and bulk solids. Governed by ISO 1496-3.
Bulk. Dry container with top loading hatches and discharge openings, for grain, resins or sugar.
Open Side. Doors along the full side, for long cargo or cargo that needs full front access.

How to read a container identification code

Here it is worth correcting a common mistake: the body that manages the worldwide register of owner codes is the BIC (Bureau International des Containers et du Transport Intermodal), based in Paris, not "IBC". The register was created by the BIC and adopted by ISO in 1972 as part of ISO 6346. An owner code is only valid if it is registered with the BIC.

A container number has eleven characters. Take the example MSCU 123456 7:

MSC: owner code. Three letters that identify the owner or principal operator, assigned by the BIC.
U: equipment category identifier. U designates freight containers; J, detachable freight container-related equipment; Z, trailers and chassis.
123456: six-digit serial number, unique within that owner's fleet.
7: check digit. It is calculated mathematically from the previous ten characters and lets terminals, depots and shipping lines automatically detect a mistyped number.

Next to the number is the four-character size and type code. The first two characters indicate length and height; the next two, the container type and its features. This code is what lets a system know, without seeing the unit, whether it is a 40 ft High Cube dry container or a 20 ft reefer.

What is the CSC plate and how does it relate to ISO standards?

The International Convention for Safe Containers (CSC), adopted in 1972 and in force since 1977 under the International Maritime Organization, requires every container intended for international transport to carry a riveted safety approval plate, usually on the left door.

The CSC plate is proof that the unit was built and tested in accordance with the strength requirements set out in the ISO standards. It shows the approval country and number, the date of manufacture, the container identification number, the maximum gross mass, the allowable stacking load and the transverse racking test value.

The convention provides for two inspection regimes:

PES (Periodic Examination Scheme). The first examination takes place before the container is five years old, and thereafter at intervals of no more than 30 months. The date of the next examination is marked on the plate or on an attached decal.
ACEP (Approved Continuous Examination Program). The operator inspects units continuously as part of normal operations, under a program approved by the administration, with a safety standard no lower than the PES. The ACEP number must appear on the plate.

A container with an illegible CSC plate or an expired examination should not be shipped. If you are going to buy a unit, always check that plate before closing the deal: it is the document that separates a container fit for export from one that is only good for storage. You can see the units available in our online shop.

What is ISO 17712?

ISO 17712 establishes uniform procedures for the classification, acceptance and withdrawal of mechanical seals for freight containers. It is the reference that customs authorities and supply chain security programs use to decide which seal is acceptable in international trade.

The standard defines three strength classes:

Indicative (I). The lowest level. It serves as evidence of opening, not as a real physical barrier.
Security (S). Intermediate level, for applications that require more than visual evidence.
High security (H). The level required in international trade. This is the class that bolt seals belong to.

Beyond mechanical strength, the current edition adds a key requirement: the seal must provide tamper evidence, that is, leave visible traces if someone tries to open it and close it again. Those traces can be color changes, texture alterations, cracks, notches or abrasions, and the manufacturer must demonstrate them to an accredited inspection body. Each seal carries a unique number that must match the one recorded on the Bill of Lading; seals are single-use and must be discarded after removal.

If you need advice on the right container and seal for your cargo, contact us.

Frequently asked questions

What is the standard size of an ISO container?

The external width is always 2.438 m (8 ft). The length is 6.058 m (20 ft) for the 20-foot and 12.192 m (40 ft) for the 40-foot. Standard height is 2.591 m (8 ft 6 in) and High Cube height is 2.896 m (9 ft 6 in). These dimensions are fixed by ISO 668.

What does the U in the container number mean?

It is the equipment category identifier defined by ISO 6346. U indicates a freight container; J corresponds to detachable container-related equipment and Z to trailers and chassis. It comes immediately after the three letters of the owner code.

Who assigns a container's code?

The BIC, Bureau International des Containers et du Transport Intermodal, based in Paris. It manages the worldwide register of owner codes within the framework of ISO 6346, and only prefixes registered with it are considered valid.

How often is a container inspected?

Under the periodic scheme of the CSC Convention, the first examination takes place before five years from manufacture and then every 30 months at most. Under an approved ACEP program, inspection is continuous within the owner's operation, with an equivalent level of safety.

What seal do I need to export?

International trade requires a class H high-security seal compliant with ISO 17712, with tamper evidence. The seal number must be recorded on the Bill of Lading and match the one verified by the customs authority at destination.

ISO containers with a valid CSC plate, graded and ready to deliver. See our shipping containers.