Shipping Container Unit Numbers

There are millions of shipping containers traveling around the world at any given time, and it may seem like a daunting task to keep track of all of them, but it's not as complicated as it may look on the surface. Similar to the license plates on cars and trucks, every shipping container is assigned a unit number that identifies who owns or manages it, and which specific container it is. Decaled on every container, in multiple places, is a prefix code, unit number, and check digit.

Prefix Code

The prefix code on containers tells who owns or manages the container. It's always four letters ending in “U”. For example, a container labeled with HLXU would be identified as a container owned by the shipping line Hapag Lloyd, just as a container with SILU would be managed by Sun Intermodal. If a container transfers ownership it is supposed to be labeled with a new prefix, but this doesn't always happen.

Unit Number

Following the prefix is the unit number. This six digit number is unique to every container and helps the owner or manager of the container track maintenance and repairs, and the general history of the shipping container.

Check Digit

The seventh and final number on every shipping container is the check digit. This number is used to detect errors in shipping container unit numbers, and is calculated off both the letters and numbers on the container.

To calculate the check digit takes three steps, with the final step being the most complicated.

First, take the four letters in the prefix and add up their corresponding numbers. A is equal to 10, B is equal to 12, and 11 and all multiples of it are omitted. See the chart below (and remember this as the first number).

The numbers are then multiplied by a number based on their order in the list as follows:

Finally, all of the numbers are added up and divided by 11. Any fractions are dropped, and this number is multiplied by 11. From here we just subtract this number from the first number and we're left with our check digit.

Example: