The international standard that defines how a shipping container is identified: a 3-letter owner code, a 1-letter category, a 6-digit serial and a check digit — 11 characters in total, plus optional size/type codes.

What are the U, J and Z category letters?

U is a freight container (the vast majority), J a detachable freight chassis, Z a trailer or chassis. The 4th letter of a container number is always one of these.

How do I know a container number is valid?

The 11th character is a check digit computed from the first ten. Use the free check-digit calculator linked below — it shows the full calculation and flags a single mistyped character.

Does ISO 6346 tell me where the container is?

No — it only identifies the box. Position comes from the shipping line. TrackJet uses the owner code to route you to the right line and, where a licensed or authorised source exists, builds a source-labelled timeline.

ISO 6346 container numbers explained

ISO 6346 is the international standard that gives every shipping container a unique, machine-readable identity. Understanding it tells you how to read a container number, how to spot a typo, and how tracking finds the right shipping line. This guide breaks down each part, with a worked example and a free validator.

The four parts of a container number

A standard ISO 6346 identifier is 11 characters — for example CSQU 3 054383 → CSQU3054383:

Owner code (3 letters) — CSQ. Registered with the Bureau International des Containers (BIC); it identifies the company that owns the container.
Category identifier (1 letter) — U. U = freight container, J = detachable freight chassis, Z = trailer or chassis. It is always one of these three.
Serial number (6 digits) — 054383. Assigned by the owner; unique within their fleet.
Check digit (1 digit) — 3. Computed from the other ten characters so a mistyped number can be caught automatically.

The check digit, worked

Each of the first ten characters becomes a number (digits as themselves; letters via the ISO 6346 table where A=10, B=12 … Z=38, skipping multiples of 11). Each is multiplied by a doubling weight — 1, 2, 4, 8 … 512 — the products are summed, and the total is taken modulo 11 (a remainder of 10 becomes 0). For CSQU3054383 the sum is 6185 and 6185 mod 11 = 3, matching the final digit. The check-digit calculator shows every row for any number you paste.

Size and type codes

ISO 6346 also defines a separate 4-character size-and-type code, often shown beneath the number — for example 45G1 (a 40-foot high-cube general-purpose container) or 22G1 (a 20-foot general-purpose container). The first character is length, the second height/width, the last two the container type. These are descriptive; they are not part of the tracking identifier.

How the number leads to tracking

The owner code is the key. TrackJet reads it, identifies the shipping line and routes you to that line’s official tracking surface — the authoritative source for the container’s position. Where the line offers a licensed feed, or you forward an update, TrackJet records a source-labelled timeline you can independently verify. It never invents an event: ISO 6346 identifies the box, but only the carrier knows where it is.

FAQ

What is ISO 6346?: The international standard that defines how a shipping container is identified: a 3-letter owner code, a 1-letter category, a 6-digit serial and a check digit — 11 characters in total, plus optional size/type codes.
What are the U, J and Z category letters?: U is a freight container (the vast majority), J a detachable freight chassis, Z a trailer or chassis. The 4th letter of a container number is always one of these.
How do I know a container number is valid?: The 11th character is a check digit computed from the first ten. Use the free check-digit calculator linked below — it shows the full calculation and flags a single mistyped character.
Does ISO 6346 tell me where the container is?: No — it only identifies the box. Position comes from the shipping line. TrackJet uses the owner code to route you to the right line and, where a licensed or authorised source exists, builds a source-labelled timeline.