The four parts of a container number
A container number under ISO 6346 is eleven characters — for example CSQU3054383 — made of four parts:
| Part | Length | Example | Meaning |
|---|---|---|---|
| Owner code | 3 letters | CSQ | The owner or principal operator, registered as a BIC code |
| Equipment category | 1 letter | U | U for freight containers, J for detachable container-related equipment, Z for trailers and chassis |
| Serial number | 6 digits | 305438 | Chosen by the owner |
| Check digit | 1 digit | 3 | Calculated from the first ten characters |
The prefix holder, the owner, a lessor, the operator and the carrier moving the box today can all be different parties. A refrigerated container is still category U — refrigeration is described by the size/type code, not the category letter.
How the ISO 6346 check digit is calculated
The check digit is a mod-11 self-check over the first ten characters. Each character is given a value: digits keep their own value, and letters run from A = 10 to Z = 38, skipping every multiple of 11 (so 11, 22 and 33 are never used, and no letter value collides with the modulus). Each value is multiplied by a weight — the powers of two from 1 to 512, left to right — the ten results are added, and the remainder after dividing by 11 is the check digit. A remainder of 10 is written as 0.
Worked example — CSQU305438: the letters are C = 13, S = 30, Q = 28, U = 32. The weighted sum C·1 + S·2 + Q·4 + U·8 + 3·16 + 0·32 + 5·64 + 4·128 + 3·256 + 8·512 is 6185, and 6185 mod 11 is 3, so the check digit is 3 and the full number is CSQU3054383. Open this example in the checker.
The size and type code
Next to the container number you will usually see a four-character size/type code such as 22G1 or 45R1. It describes the container's dimensions and kind and, unlike the number, has no check digit. See container size and type codes for how to read it.
What a valid ISO 6346 number does not prove
A correct check digit confirms only that the printed digit agrees with the first ten characters under the standard's arithmetic. It does not prove that the container exists, who owns or operates it, where it is, or that anyone may release it — and it cannot catch every mistake, because some errors are invisible to the check digit.