ISO 6346: How to Read a Container Number

Every container has a number on its door in the same layout: four letters, six digits and a single digit that is usually boxed. This layout is defined by the ISO 6346 standard. Every part of the number tells you something.

The four parts of the number

Example: CSQU 305438 3

1. Owner code (3 letters) — CSQ: Identifies the owner or operator of the container. These codes are registered by the Paris-based Bureau International des Containers (BIC); the same code is never issued to two companies.
2. Equipment category identifier (1 letter) — U: U means freight container, J detachable equipment attached to a container, and Z trailer and chassis.
3. Serial number (6 digits) — 305438: The number the owner assigns within its own fleet.
4. Check digit (1 digit) — 3: Calculated from the first ten characters. It exists to catch errors when the number is written or read incorrectly.

How is the check digit calculated?

1. Convert the letters to numbers. Start at A=10, but skip multiples of 11: A=10, B=12, C=13 … K=21, L=23 … U=32, V=34 … Z=38. Digits keep their own value.
2. Multiply each character by the weight of its position. The weights are 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 (that is, 2 to the power 0 to 9).
3. Add up the products and find the remainder of the sum divided by 11.
4. The remainder is the check digit. If the remainder is 10, the check digit is taken as 0.

You don’t have to do this calculation by hand: the Check Digit Calculator validates the number step by step with a table. The calculation runs in your browser and no data is stored.

Size and type code

Below or next to the number there is a second, four-character code. For example 22G1 or 45G1:

• 1st character, length: 2 = 20 ft, 4 = 40 ft, L = 45 ft
• 2nd character, height: 2 = 8 ft 6 in (standard), 5 = 9 ft 6 in (high cube)
• 3rd–4th characters, type: G1 = general purpose (dry), R1 = mechanically refrigerated (reefer), U1 = open top

Accordingly, 22G1 is a standard 20 ft dry container, 45G1 a 40 ft high cube dry, and 45R1 a 40 ft high cube reefer. To see the types in a 3D model, try the 3D Container Viewer.

Why does the number matter?

The bill of lading, customs declaration, lease agreement and examination certificate all rely on the same number. A single wrong character can cause the container to go "missing" at the port or the documents to be rejected. The most common mix-ups are between 0/O and 1/I. The check digit catches most of these errors.