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How the container number check digit works (ISO 6346).

4 min read · Updated 4 November 2026

Every shipping container number ends in a check digit. It's calculated from the other ten characters, so if one character is mistyped, the check digit almost always stops matching. It's the fastest way to catch a typo before it reaches the bill of lading.

How a container number is built

A container number has 11 characters, for example MSKU 123456 5:

  • 3 letters: the owner code, registered with the BIC
  • 1 letter: the equipment category (U for freight containers, J for detachable equipment, Z for trailers and chassis)
  • 6 digits: the serial number
  • 1 digit: the check digit

How the check digit is calculated

Each letter is turned into a number: A is 10, B is 12, and so on up to Z, skipping 11, 22 and 33. Digits keep their own value.

Each of the first ten values is multiplied by 2 raised to its position (1, 2, 4, 8 … 512). The results are added up, the total is divided by 11, and the remainder is the check digit. A remainder of 10 counts as 0.

What the check digit catches, and what it doesn't

It catches almost every single-character typo and most swapped neighbours, which are the usual mistakes when re-typing from a PDF.

It doesn't tell you the container is the right one for this booking. A valid number can still belong to a different shipment, so also compare container numbers across the SI, the VGM and the booking.

Free tools for this

Free toolContainer number checkerCheck one container number or a whole list against the ISO 6346 check digit.Open →

Questions

Can I check a container number for free?
Yes. Use the free container number checker on this site: type the number and it shows the correct check digit.
Why does a remainder of 10 become 0?
The check digit has to be a single digit, so ISO 6346 maps 10 to 0. Owners usually avoid serial numbers that produce 10 for this reason.

Let LOGIDOC do the checking

LOGIDOC reads every SI, VGM and booking confirmation from your inbox, flags the errors before the cut-off, and lays the approved data out for the carrier's website. Book a 30-day pilot or try the demo.

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