9 minute guide
ISO 6346 Container Check Digit: Formula & Example
Understand the ISO 6346 container-number format, letter values, modulo-11 check digit calculation and validation limits.
ISO 6346 check digit in brief
An ISO 6346 container number contains a three-letter owner code, one equipment-category letter, a six-digit serial number and one check digit. The check digit is calculated by mapping letters to ISO values, applying powers-of-two weights and reducing the sum modulo 11.
Container number format
A typical freight-container identifier has eleven characters. The first three letters identify the owner code, the fourth is the equipment-category identifier—commonly U for freight containers—the next six digits are the serial, and the last digit is the check digit. Printed spaces are not part of the number.
Letter values and position weights
Letters use numeric values beginning with A=10 while skipping multiples of 11. Starting at the leftmost character, each value is multiplied by 2 raised to its zero-based position: 1, 2, 4, 8 and so on. Digits retain their numeric value.
Check digit formula step by step
Convert the first ten characters to values, multiply each by its position weight, add the products, and divide the sum by 11. The remainder becomes the check digit; a remainder of 10 is represented as 0.
Worked validation example
For an operational check, use the container-number validator on this site. It performs the same deterministic calculation, shows whether the printed final digit agrees, and proposes the expected full number when it does not.
What a failed or valid check digit means
A failure usually indicates a transcription or OCR error, so compare the number with the container door and source document. A passing digit confirms arithmetic structure only; it does not prove ownership, availability, physical condition, location or shipment status.
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