Container number check (ISO 6346)
Type a container number to verify its check digit, owner code and equipment category. See the calculation step by step, check a whole list at once and decode size-type codes such as 22G1 or 45G1.
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How a container number is built
Every intermodal freight container carries a unique identification code defined by ISO 6346. It has 11 characters, for example CSQU 305438 3:
| Part | Example | Meaning |
|---|---|---|
| Owner code | CSQ | Three letters registered by the container owner with the BIC |
| Category identifier | U | U — freight container, J — detachable equipment (e.g. a clip-on generator), Z — trailer or chassis |
| Serial number | 305438 | Six digits assigned by the owner |
| Check digit | 3 | Calculated from the first ten characters to catch typos |
How the check digit is calculated
- Convert letters to numbers: A = 10, B = 12, C = 13 … Z = 38. Multiples of 11 (11, 22, 33) are skipped, so K = 21, L = 23, U = 32, V = 34.
- Multiply the value of each of the first 10 characters by 2 to the power of its position (1, 2, 4, 8 … 512).
- Add the results, divide by 11 and take the remainder. A remainder of 10 gives check digit 0.
= 13 + 60 + 112 + 256 + 48 + 0 + 320 + 512 + 768 + 4096 = 6185
6185 mod 11 = 3 → CSQU 305438 3
The check digit catches almost all single-character typos and most swapped digits, which is why it is worth validating numbers before you send a booking, a customs declaration or a tracking request.
Size-type codes
Next to the number you will often see a four-character size-type code. The first character is the length (2 = 20 ft, 4 = 40 ft, L = 45 ft), the second the height (2 = 8 ft 6 in, 5 = 9 ft 6 in high cube), then the type group (G = general purpose, R = reefer, U = open top, P = flat rack, T = tank). So 22G1 is a standard 20′ dry container and 45G1 a 40′ high cube.
Frequently asked questions
How do I check if a container number is valid?
Enter all 11 characters above. The tool converts the letters to numbers, multiplies each character by a power of two, takes the sum modulo 11 and compares the result with the last digit. If they match, the number is well-formed under ISO 6346.
What does the U in a container number mean?
The fourth letter is the equipment category. U means a freight container, J means detachable freight-container-related equipment and Z means a trailer or chassis. Almost all shipping containers you track will have U.
What if the check digit is wrong?
Most often it is a typo — a mistyped letter, O instead of 0, or two swapped digits. Compare with the documents or the photo of the container doors. A wrong check digit means the number cannot be correct as written.
Does a valid check digit prove that the container exists?
No. It only proves the number is correctly formed. Whether the owner code is registered is recorded by the BIC, and the container's location is known only to the shipping line or terminal.
Can I find out which shipping line owns a container?
The first four letters (owner code plus category) identify the owner. The tool recognises prefixes of major lines such as MSC, Maersk, CMA CGM, Hapag-Lloyd, COSCO, Evergreen and ONE. For any other prefix, check the BIC register. Note that many containers are leased, so the owner is not always the carrier.
Can I track containers in milog?
In milog you keep container numbers with your shipments, see statuses and ETA on a map and send your clients a tracking link with email and Telegram updates. Create a free account — free for everyone until September 2027.
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