ISO 6346 Check-Digit Calculator

Container number structure

• OWL3-letter owner code (BIC-registered).
• UEquipment category — U for freight containers (the common case), J for detachable freight-related equipment, Z for trailers and chassis.
• 1234566-digit serial (owner-assigned).
• 3Check digit (computed from the first 10 characters).

The algorithm in five lines

1. Look up each of the first 10 characters' value (letters per the table below, digits are themselves).
2. Multiply each value by 2 to the power of its position (positions 0–9).
3. Sum all ten products.
4. Take that sum modulo 11.
5. The remainder modulo 10 is the check digit. (A remainder of 10 is printed as 0; ISO 6346 recommends not allocating those serials, since single-character errors can slip past the check.)

Letter → integer table

Letters skip multiples of 11 (the gaps before L and V). That guarantees no two letters are congruent mod 11, so a single substitution always changes the check digit.