A container number is not a serial number with letters stuck on the front. Four of its eleven characters identify the owner, one identifies what kind of equipment it is, six are the owner’s own serial, and the last is a check digit that you can calculate by hand. If the check digit does not match, the number is wrong — and that is worth knowing before it goes onto a shipping bill.
The standard, and who runs it
Container marking is governed by ISO 6346. Owner prefixes are registered with the Bureau International des Containers (BIC) in Paris, which is the registrar appointed for the standard. Because prefixes are registered rather than invented, no two owners share one, and a prefix is a reliable pointer to the owner of the box — though not, as we will come to, to the carrier moving it.
The four parts of the number
Take CSQU 305438 3. It splits like this.
| Part | Characters | In this example | Meaning |
|---|---|---|---|
| Owner code | 3 letters | CSQ | The registered owner or principal operator |
| Category identifier | 1 letter | U | What class of equipment this is |
| Serial number | 6 digits | 305438 | The owner’s own reference, assigned freely |
| Check digit | 1 digit | 3 | Calculated from the ten characters before it |
The owner code is always three letters and the fourth letter is the category. People habitually read the first four letters as one block — MAEU, MSCU, TGHU — which is fine in practice, but it hides the fact that the fourth character is carrying separate information.
The category identifier: U, J and Z
- U — a freight container. This is what you will see on essentially every box in a stack.
- J — detachable freight-container-related equipment. The clearest example is a clip-on generator set for a reefer, which is a separate item that travels with a container but is not the container.
- Z — trailers and chassis. Road equipment that carries containers rather than a container itself.
If you are looking at a number whose fourth letter is not U, you are almost certainly not looking at a shipping container, and it should not be entered as one on a document. Tank containers, reefers, flat racks and open tops are all still U — the equipment type is recorded separately, in the size and type code described below.
The check digit, worked out in full
This is the part most people skip, and it is the part that catches keying errors before they reach a customs system. The rule has three steps.
Step 1: give every character a value
Digits are worth their face value: 0 is 0, 7 is 7. Letters start at A = 10 and count upwards, but every multiple of 11 is skipped. So 11, 22 and 33 are never used. That gives:
| Letter | Value | Letter | Value | Letter | Value |
|---|---|---|---|---|---|
| A | 10 | J | 20 | S | 30 |
| B | 12 | K | 21 | T | 31 |
| C | 13 | L | 23 | U | 32 |
| D | 14 | M | 24 | V | 34 |
| E | 15 | N | 25 | W | 35 |
| F | 16 | O | 26 | X | 36 |
| G | 17 | P | 27 | Y | 37 |
| H | 18 | Q | 28 | Z | 38 |
| I | 19 | R | 29 |
Note where the skips fall: 11 is skipped between A and B, 22 between K and L, 33 between U and V. That is why the letter values are not simply A = 10 through Z = 35.
Step 2: weight each character by its position
Each of the first ten characters is multiplied by 2 raised to the power of its position index, counting from zero. So the first character is multiplied by 2 to the power 0, which is 1; the second by 2; the third by 4; and so on to the tenth, which is multiplied by 2 to the power 9, which is 512.
Step 3: divide the total by 11
The remainder is the check digit. If the remainder is 10, it is written as 0.
The arithmetic for CSQU3054383
| Position | Character | Value | Weight | Product |
|---|---|---|---|---|
| 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 divided by 11 gives 562 with 3 left over: 11 × 562 = 6182, and 6185 − 6182 = 3. The check digit is 3, which matches the number as printed. CSQU3054383 is valid.
What the check digit does and does not catch
Running the same calculation across every possible single-character substitution in this number, the check digit catches all but a handful. The gap comes from writing remainder 10 as 0, which means two different remainders print as the same digit. Every adjacent transposition in this example is caught. In practice the check digit is a strong filter, not a proof: a number that fails is definitely wrong, while a number that passes is very probably right.
The size and type code on the door
Below the container number you will see a separate four-character code — 22G1, 45G1, 42R1 and so on. This describes the box, not its identity, and it is the code that actually tells you what you have booked.
- First character — length. 2 is 20 ft, 4 is 40 ft, L is 45 ft.
- Second character — height and width. 0 is 8 ft 0 in, 2 is 8 ft 6 in, 5 is 9 ft 6 in. Letters in this position indicate widths greater than the standard 2,438 mm, which are rare in liner trade.
- Third and fourth characters — type and any distinguishing feature. G is general purpose, R is a refrigerated container, U is open top, P is a platform or flat rack, T is a tank. The fourth character records a variation within that type.
So 22G1 is a 20 ft general-purpose box at standard 8 ft 6 in height. 45G1 is the one that catches people out: the 4 is the length, 40 ft, and the 5 is the height, 9 ft 6 in. It is a 40 ft high cube, not a 45 ft container. A genuine 45 ft high-cube dry box is L5G1. We have seen booking amendments and, worse, a shipping bill go out on the assumption that 45G1 meant forty-five feet.
The extra 30 cm of height on a high cube is useful only if your cargo can use it. Dense cargo out of Gujarat — tiles, granite, chemicals in drums — hits the payload limit long before it fills the cube, and a high cube adds nothing but a slightly higher freight rate. Work the volume first with our CBM calculator, then check it against the payload plate on the door. Equipment selection is one of the few decisions in ocean freight that you control completely.
Where the number is used, and where it goes wrong
The container number appears on the bill of lading, the shipping bill, the packing list, the certificate of origin and the carrier’s manifest. It is one of the few fields that must be identical across every one of them. A single wrong character on a shipping bill is a correction and, depending on the stage, an amendment charge. That is the same failure pattern as most other customs clearance delays: not a rule broken, just two documents that do not agree.
Two practical points. Photograph the door of the container at stuffing, with the number and the size/type code both readable in one frame — it settles arguments later at no cost. And when you are verifying a number against tracking, remember the container number and the bill of lading number are different things; if you need to confirm which is which, the glossary has both, and our tracking page accepts either.
Frequently asked questions
Does the container prefix tell me which shipping line is carrying it?
No. It tells you who owns the box. A great many containers are owned by leasing companies, so a container with a leasing prefix can move under any carrier’s bill of lading. Even a carrier-owned box can be sub-leased. Use the bill of lading to identify the carrier, not the container.
What does the fourth letter U mean on a container number?
It is the ISO 6346 category identifier for a freight container. J marks detachable container-related equipment such as a clip-on generator, and Z marks trailers and chassis.
Can two containers have the same number?
They should not. Prefixes are uniquely registered and each owner controls its own serial range. Duplicates do occasionally appear, usually from a box that was scrapped and a serial later reissued, or from unregistered equipment. If a number tracks to a movement that makes no sense, a duplicate is one explanation worth considering.
Is the check digit ever a letter?
No. It is always a single digit, 0 to 9. A remainder of 10 is written as 0.