How to read a container number
Every shipping container has a unique 11-character identifier stamped on all four sides. Once you know the pattern, you can decode any container number in seconds — and understand exactly why it matters for tracking.
The four-part format
Every intermodal container has a unique ID that follows the same global standard (ISO 6346). The format is always: three letters, one letter, six digits, then a hyphen and one final digit. For example: MSCU 743210-5.
Break it apart and you get four pieces: an owner code (3 letters), an equipment category identifier (1 letter), a serial number (6 digits), and a check digit (1 digit). Together they make the container globally unique for its entire working life — often 20+ years.
The pattern
OWNER (3 letters) + CATEGORY (1 letter) + SERIAL (6 digits) + CHECK (1 digit) = 11 characters total
The owner code
The first three letters identify who owns the container — not necessarily the shipping line moving it. Many carriers lease boxes from specialist leasing companies, so the owner code on the side of a Maersk ship might belong to Triton or Textainer.
The fourth character is the equipment category identifier. For standard freight containers it is always U. You might occasionally see J (for detachable freight equipment like flat-racks) or Z (for trailers), but the vast majority of containers you will ever track end in U.
Owner codes are registered through the Bureau International des Containers (BIC), which ensures each three-letter prefix is unique worldwide.
The serial number
The six digits after the owner code are a serial number assigned by the owner. They carry no inherent meaning — two containers with sequential serial numbers might be on opposite sides of the world at any given moment.
What matters is that the combination of owner code + serial is globally unique. No two containers in active service share the same full number.
The check digit
The last digit — written after the hyphen — is a check digit calculated from all ten preceding characters. Its only job is to catch typos and scanning errors.
The calculation converts each letter to a number, weights every character by its position (powers of 2), sums everything, divides by 11, and the remainder is the check digit. If the result is 10, the check digit is written as 0.
Why this matters
Most tracking software and APIs validate the check digit automatically. If you mistype a single character in a container number, the check digit mismatch is caught before any API request is made.
Why the container number matters for tracking
The container number is the primary key the entire logistics chain uses to identify a box. Every terminal scan, vessel manifest, customs entry, and carrier booking references it — from the first gate-in at origin to the final gate-out at destination.
When you call a container tracking API, you send this number. The API uses the owner prefix to identify the carrier, queries terminal and vessel data, and assembles a complete event timeline — all from one 11-character string.
Frequently asked questions
What if I only have a bill of lading number?
Most container tracking APIs accept a bill of lading (B/L) number and resolve the associated container number automatically. You do not need the container number directly.
Can two containers share the same number?
No. The BIC registration system ensures owner codes are globally unique and owners assign unique serials within their prefix. The check digit adds one more layer of validation.
What does the U in position four mean?
U stands for freight container — the standard intermodal box. Other codes (J, Z) exist for specialised equipment, but nearly every container you track will be a U.
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