ISO 6346 Container Check Digit: How It Is Calculated
A container number is an owner code, an equipment category, a six-digit serial, and a single ISO 6346 check digit computed from a weighted sum modulo 11.
Updated 2026-10-01 · 4 min read
What a container number is
An ISO 6346 container number has four parts: a three-letter owner code, a one-letter equipment category, a six-digit serial number, and a single check digit.
The owner code identifies the operator, the category says what kind of equipment it is, and the check digit catches transcription errors.
The letter values
Letters are mapped to numbers using a table that skips multiples of 11 (11, 22, 33): A is 10, B is 12, C is 13, and so on to Z as 38. Digits keep their value.
The calculation
Each of the first ten characters is multiplied by a power of two, starting at 1 for the first character and doubling each position. The ten products are summed, taken modulo 11, and the remainder is the check digit — except a remainder of 10, which is written as 0.
For CSQU3054383, the weighted sum is 478, 478 mod 11 is 5, and the check digit is 3 because the sum including the check digit must be divisible by 11.
What it does not prove
A valid check digit means the number is internally consistent. It does not confirm that the container exists, who owns it, or where it is. Those live with the operator and the carrier.
Try it on real data
Frequently Asked Questions
Eleven: three owner letters, one category letter, six serial digits, and one check digit.
A weighted sum modulo 11, with weights that double each position and letter values that skip multiples of 11.
It is the equipment category: U for freight containers, J for detachable equipment, and Z for trailers and chassis.