ISO 6346 Container Check Digit Calculation

Letter values

Letters map to numbers starting at A=10 and skipping multiples of 11: A=10, B=12, C=13 … K=21, L=23 … U=32, V=34 … Z=38. Digits keep their value.

Extracting It Automatically

When you read this data from photos, the Container Number OCR API returns it as structured JSON, and you can apply the rules on this page as validation. Try it with the free scanner. To check a list of numbers without writing code, use the free container check digit calculator.

Where This Fits

• Weights: Multiply the value at position i (0–9) by 2 to the power of i, then add the products.
• Remainder: Take the sum modulo 11, then modulo 10. The result is the check digit, so a remainder of 10 becomes 0.

Python Implementation

A minimal, dependency-free implementation you can use to validate OCR output.

Prerequisite: Python 3.9+

def letter_values():
    values, n = {}, 10
    for char in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
        if n % 11 == 0:
            n += 1
        values[char] = n
        n += 1
    return values

VALUES = {**letter_values(), **{str(d): d for d in range(10)}}

def iso6346_check_digit(prefix10: str) -> int:
    total = sum(VALUES[c] * (2 ** i) for i, c in enumerate(prefix10.upper()))
    return total % 11 % 10

print(iso6346_check_digit("CSQU305438"))  # -> 3

Reference Summary

Key facts from this page in structured form, for quick lookup.

{
  "Letter values": "Letters map to numbers starting at A=10 and skipping multiples of 11: A=10, B=12, C=13 … K=21, L=23 … U=32, V=34 … Z=38.",
  "Weights": "Multiply the value at position i (0–9) by 2 to the power of i, then add the products..",
  "Remainder": "Take the sum modulo 11, then modulo 10."
}