Three layers of marking on each box
The first layer is identification: owner code, category letter, serial number and check digit, in characters at least 100 mm (4 in) tall. The second is the size and type code, which commonly sits right under the ID. The third is operational marks: maximum gross and tare mass, a height mark on boxes taller than 2.6 m (8 ft 6 in), and an overhead electrical danger sign on containers fitted with ladders.
Decoding 22G1, 45G1 and similar codes
The first character gives the length, where 2 means 20 feet and 4 means 40 feet. The second gives the height, where 2 means 8 ft 6 in and 5 means a 9 ft 6 in high cube. The last two characters give the type, such as G1 for general purpose, R1 for refrigerated, U1 for open top and T1 for tank. An easy slip is reading 45G1 as a 45-foot box. It is a 40-foot high cube.
Standards people confuse with it
ISO 6346 covers codes and markings, not the box itself. External dimensions and ratings come from ISO 668, testing requirements from the ISO 1496 series, and corner fittings from ISO 1161. The safety approval plate comes from a separate treaty, the International Convention for Safe Containers. BIC lists the latest edition of ISO 6346 as published in August 2022, and it keeps the owner-code register the standard relies on.
Example
The side of a high cube shows HLBU2456904, the code 45G1 just below it, and a yellow-and-black height mark near the top right corner, all placed as ISO 6346 describes.
Track a container
Save the reference now and create your account; tracking starts once your 30-day trial is active.
Official sources
Questions
What does ISO 6346 mean?
ISO 6346 is the international standard for coding, identifying and marking freight containers. It defines the 11-character container ID, the four-character size and type code, and the operational marks, such as maximum gross mass and height warnings, shown on each box.
Is ISO 6346 mandatory?
ISO standards are voluntary in themselves, yet carriers, lessors, terminals and customs systems all rely on ISO 6346 identification, so boxes in international trade carry it as a matter of course.
Where can I read the full standard?
ISO sells the standard. BIC's website summarizes the identification system, size and type codes and operational marks, which covers most day-to-day questions about reading a container.