Container Numbers Explained: ISO 6346 and Check Digits
2026-10-06 · HS Code Atlas
A shipping container number identifies equipment, not a purchase order or a whole cargo booking. It is normally visible on the container and on transport documents. Copying the entire identifier makes it possible to distinguish one box from other equipment with a similar serial.
The four parts of the identifier
The BIC identification guide describes the ISO 6346 structure. Using the illustrative number MSKU1234565:
| Part | Example | Meaning |
|---|---|---|
| Owner code | MSK | Three-letter code associated with the registered owner or principal operator |
| Equipment category | U | Category identifier; U is used for freight containers |
| Serial | 123456 | Six-digit equipment serial, including leading zeros |
| Check digit | 5 | A digit used to check the preceding characters |
Related category letters include J for detachable container-related equipment and Z for trailers or chassis. A routine freight-container search usually uses a U identifier. Do not substitute a seal number, trailer plate or booking reference.
What the check digit does
ISO 6346 uses assigned letter values and positional weights to derive a check digit from the first ten characters. Universal Container Tracking performs that format check in your browser and warns when the final digit does not match.
A warning is a reason to compare the number with a photo or original document. Look for transposed serial digits, missing leading zeros or a mistaken letter. Do not invent a replacement number simply to remove the warning; ask the sender or forwarder if the document itself appears inconsistent.
A valid check digit is not a container-location result. It does not confirm that equipment is booked, loaded, customs-cleared or even associated with your shipment.
Why the owner code is not always the carrier
The equipment owner and the carrying shipping line can differ. Containers can be leased or operated under arrangements in which an owner-code suggestion is not the correct tracking line. Confirm the carrier on the booking confirmation or bill of lading.
For example, a familiar prefix may suggest Maersk or MSC, but the transport document should decide which portal you use. An unfamiliar owner code can still be a correctly formed container number. The universal tool offers manual selection for supported shipping lines.
Equipment tracking versus shipment tracking
A single container may move under a larger shipment reference, and a booking may include multiple boxes. Use the container number to inspect equipment-specific milestones. Use the carrier’s bill-of-lading or booking mode for shipment-level questions when that mode is supported.
Read container number vs bill of lading vs booking number before changing reference types. For a named line, use the Maersk or MSC tracking guide. These pages use the same universal tool and open official carrier records.