bol.aiContainer number check digit calculator
Paste a shipping container number — we'll tell you if the checksum adds up.
Nothing is sent anywhere — the check runs entirely in your browser.
The anatomy of a container number
Every intermodal shipping container carries a unique identifier standardised in ISO 6346: four letters, then seven digits. It breaks into four parts — and the last one keeps the other three honest.
The first three letters are the owner code, registered with the BIC (Bureau International des Containers). The fourth letter is the category — U for freight containers, J for detachable equipment, Z for chassis. Then a six-digit serial number, and finally the check digit, shown in amber because it is the one character that proves the rest were copied correctly.
How the check digit is calculated
The check digit isn't arbitrary — it's a checksum derived from the other ten characters, which is why a single mistyped character almost always breaks it.
1 — Convert the letters to numbers
Each letter has a value from 10 to 38, assigned in order but skipping every multiple of 11 (11, 22, 33), because those would collide with the final step:
| 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 |
2 — Weight each character by position
Multiply each of the ten characters by 2 raised to its position: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512.
3 — Sum, then take modulo 11
Add the weighted values and divide by 11. The remainder is the check digit — and a remainder of 10 is written as 0.
Worked example: MSKU 431829
| M | 24 × 1 | 24 |
| S | 30 × 2 | 60 |
| K | 21 × 4 | 84 |
| U | 32 × 8 | 256 |
| 4 | 4 × 16 | 64 |
| 3 | 3 × 32 | 96 |
| 1 | 1 × 64 | 64 |
| 8 | 8 × 128 | 1024 |
| 2 | 2 × 256 | 512 |
| 9 | 9 × 512 | 4608 |
| Sum | 6792 | |
| 6792 mod 11 | = 5 | |
The check digit is 5, so the full number is MSKU4318295. Run it through the calculator above to confirm.
The AI guesses; the checksum knows.
This is the same check we run on every extraction
Bol.ai reads Bills of Lading and returns structured data — and validates every container number against ISO 6346 as it goes. When the AI misreads a character on a smudged scan, the checksum catches it and the field is flagged, not silently trusted.
Extract a Bill of Lading →Frequently asked questions
What is a container number check digit?
It is the single number at the end of a container number (the 11th character), calculated from the other ten under ISO 6346. Because it is derived mathematically, a typo in the owner code or serial number can be detected instantly.
What does a container number look like?
Four letters then seven digits, e.g. MSKU 431829 5: three-letter owner code, one category letter (U, J or Z), a six-digit serial number, and the check digit.
Why would a valid-looking number fail the check?
Almost always a human typo when copying the number, or an OCR/AI system that misread a character on a scanned document. Most single-character errors break the checksum — which is exactly the point.
Does a passing check digit mean the container is real?
No — it only means the number is well-formed. The check digit proves the number is internally consistent, not that a container with that number exists or sits on a particular vessel. It is a typo-catcher, not a registry lookup.