About this calculator
Every sea container carries an 11-character identification number under ISO 6346, for example CSQU 305438 3: a three-letter owner code, one equipment category letter (U for freight containers, J for detachable equipment such as clip-on gensets, Z for trailers and chassis), a six-digit serial number and a check digit. The number appears on the booking, Bill of Lading, VGM, customs documents and terminal gate-in.
The check digit is calculated from the first 10 characters so that typing errors are caught before they cause trouble. A single swapped letter or digit on a document can leave a container on hold, force a document amendment or make cargo miss the vessel. Export clerks, forwarders and warehouse staff often copy numbers from photos or handwriting, so a quick check saves time.
How it works: each letter gets a value (A = 10, then one more per letter while skipping 11, 22 and 33, so B = 12 and Z = 38) and each digit counts as itself. The values in positions 0 to 9 are multiplied by 1, 2, 4, 8 up to 512, added up and divided by 11. The remainder is the check digit, and a remainder of 10 is written as 0. The calculator accepts spaces, dashes or dots and shows the full calculation table. A number that passes is not necessarily registered or real; official owner codes are registered with the BIC, and container details must still be matched with the carrier's documents.
Worked example
Working out the check digit for CSQU 305438:
| Position | Character | Value | × Weight | Result |
|---|---|---|---|---|
| 0 | C | 13 | × 1 | 13 |
| 1 | S | 30 | × 2 | 60 |
| 2 | Q | 28 | × 4 | 112 |
| 3 | U | 32 | × 8 | 256 |
| 4 | 3 | 3 | × 16 | 48 |
| 5 | 0 | 0 | × 32 | 0 |
| 6 | 5 | 5 | × 64 | 320 |
| 7 | 4 | 4 | × 128 | 512 |
| 8 | 3 | 3 | × 256 | 768 |
| 9 | 8 | 8 | × 512 | 4,096 |
- Sum: 6,185
- 6,185 ÷ 11 = 562 remainder 3 (11 × 562 = 6,182)
The check digit is 3, so the full valid number is CSQU 305438 3.
How to use it
Type the container number
Enter all 11 characters, for example CSQU 305438 3. Spaces, dashes and dots are allowed and ignored.
Or compute the check digit
Enter only the first 10 characters (4 letters + 6 digits) to get the check digit they should have.
Read the validation
See whether the owner code, U/J/Z category, serial number and check digit are correct, with a step-by-step calculation table.