How a container number is built
Container numbers follow ISO 6346, the international standard for identifying freight containers. Every number has 11 characters in four parts. Using CSQU 305438 3 as an example:
- Owner code — CSQ
- Three letters registered with the BIC (Bureau International des Containers) in Paris. Each code belongs to one company — a shipping line, a leasing company, or another owner.
- Category — U
- One letter: U for a freight container, J for detachable equipment that goes with a container (such as a clip-on generator), Z for a trailer or chassis. Almost every box you see is U.
- Serial number — 305438
- Six digits chosen by the owner. Together with the owner code it identifies one container.
- Check digit — 3
- One digit calculated from the first ten characters. If any of those is typed wrong, the check digit usually stops matching.
Shipping lines and carrier registries often talk about the “prefix” — the owner code plus the category letter, such as MSKU. That four-letter prefix is what our tracking pages use to recognise which carrier a number belongs to.
How the check digit is calculated
- Turn each letter into a number using the table below. Digits keep their own value. The letter values start at A = 10 and skip 11, 22 and 33 (multiples of 11), which is why B is 12 and V is 34.
- Multiply each of the first ten characters by its position weight: 1, 2, 4, 8, 16 … 512 (each one double the last).
- Add up the ten results.
- Divide the total by 11 and keep the remainder. That remainder is the check digit — except a remainder of 10, which is written as 0.
Letter values
| A | B | C | D | E | F | G | H | I | J | K | L | M |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 10 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 23 | 24 |
| N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
| 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 34 | 35 | 36 | 37 | 38 |
Worked example: CSQU3054383
| Position | Character | Value | Weight | Value × weight |
|---|---|---|---|---|
| 1 | C | 13 | 1 | 13 |
| 2 | S | 30 | 2 | 60 |
| 3 | Q | 28 | 4 | 112 |
| 4 | U | 32 | 8 | 256 |
| 5 | 3 | 3 | 16 | 48 |
| 6 | 0 | 0 | 32 | 0 |
| 7 | 5 | 5 | 64 | 320 |
| 8 | 4 | 4 | 128 | 512 |
| 9 | 3 | 3 | 256 | 768 |
| 10 | 8 | 8 | 512 | 4096 |
| Total | 6185 | |||
6185 ÷ 11 = 562 remainder 3 (because 11 × 562 = 6182, and 6185 − 6182 = 3). So the check digit is 3 and the full container number is CSQU3054383.
When the remainder is 10
Take MSKU100009. Its weighted total is 5048, and 5048 ÷ 11 leaves a remainder of 10. A check digit has to be a single digit, so ISO 6346 writes a remainder of 10 as 0: the full number is MSKU1000090. The downside is that 0 then stands for two different remainders, so a mistake is slightly less likely to be caught. The standard advises owners not to issue serial numbers that give a remainder of 10, but some containers in service still have them — so a check digit of 0 can be perfectly valid.
Common reasons a container number fails
- Letter O and zero mixed up. The first four characters are always letters and the last seven are always digits, so an O can only appear in the first four.
- Check digit left off. Some paperwork prints the check digit separately or in a box. Type only ten characters here and we will tell you what the eleventh should be.
- Two digits swapped. The doubling weights mean swapping two neighbouring characters almost always changes the check digit, so the checker will flag it.
- Copied from a bill of lading or booking number. Those are different numbers. A container number is always four letters plus seven digits.
Once the number checks out, find the owner in our container tracking directory or paste it on the homepage and we will send you to the right shipping line’s tracking page.