The ISO 6346 Check Digit Algorithm
The check digit is a single-digit verification value used to catch typing errors in a container number. It is derived mathematically from the first 10 characters of the number; if even a single character is mistyped, the verification fails.
Assign values to letters
From A=10 to Z=38; multiples of 11 (11, 22, 33) are skipped — so no letter value creates ambiguity in mod 11.
Multiply by weight
Each character is multiplied by a weight based on its position: 2⁰, 2¹, 2²… 2⁹ (from 1 to 512).
Add up, divide by 11
The sum of the products is divided by 11; the remainder is the check digit.
Special case: 10 → 0
If the remainder is 10, the check digit is written as 0. Since this shares the same digit as a remainder of 0, it is recommended in practice to avoid such serial numbers.
Anatomy of the number
MSCU 654321 0
Owner code (3 letters registered with BIC) · category (U: freight container, J: detachable equipment, Z: trailer/chassis) · 6-digit serial number · check digit.
How Is the Check Digit of CSQU 305438 Calculated?
The check digit is found as follows: each of the first 10 characters is converted to a number, multiplied from left to right by the weights 1, 2, 4, 8 … 512, the products are added up, and the remainder of the total divided by 11 is taken (a remainder of 10 is written as 0). The example number commonly used to explain the standard is worked through below.
| Character | Value | Weight (2ⁿ) | Product |
|---|---|---|---|
| C | 13 | 1 | 13 |
| S | 30 | 2 | 60 |
| Q | 28 | 4 | 112 |
| U | 32 | 8 | 256 |
| 3 | 3 | 16 | 48 |
| 0 | 0 | 32 | 0 |
| 5 | 5 | 64 | 320 |
| 4 | 4 | 128 | 512 |
| 3 | 3 | 256 | 768 |
| 8 | 8 | 512 | 4096 |
Total 6185; 6185 ÷ 11 = 562, remainder 3. The check digit is 3 — the full number is CSQU 305438 3.
Letter value table (ISO 6346)
Values start at A=10; multiples of 11 (11, 22, 33) are skipped, so B=12, L=23 and V=34. Digits keep their own value.
Frequently Asked Questions About Check Digits
What should I do if the check digit does not match?
First check the number against the plate letter by letter — the most common cause is a typing error (such as mixing up 0/O or 1/I). If the number on the plate is the same and verification still fails, a professional inspection of the CSC plate and the container’s identification is recommended.
If my owner code is not registered with BIC, is the number invalid?
The mathematical verification still works; however, for international transport the owner code must be registered with BIC (Bureau International des Containers). This tool only verifies the check digit; it does not look up BIC registration.
Is this tool free, and is my data stored?
It is completely free, and the calculation runs on your device, inside your browser — the number you enter is never sent to any server and is not stored.
How many characters does a container number have?
Eleven: a 3-letter owner code, a 1-letter equipment category (U for freight containers), a 6-digit serial number and a 1-digit check digit. Example: CSQU 305438 3.
Can I track a container with this tool?
No. The tool only validates the format and the check digit of the number; it does not show where the container is. For location tracking, use the tracking page of the shipping line or the terminal.
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