The Complete Code 128 Barcode Guide

What is Code 128?

Code 128 is a high-density linear barcode symbology capable of encoding all 128 characters of ASCII. Unlike older formats, it uses variable-width bars and spaces to pack a significant amount of data into a compact horizontal footprint.

Character Sets A, B, and C

Code 128 utilizes three distinct character subsets to optimize data density:

• Subset A: Alphanumeric characters, punctuation, and control codes (like CR/LF).
• Subset B: Standard alphanumeric characters, punctuation, and lowercase letters.
• Subset C: Double-density numeric encoding. It packs two digits into a single symbol, making long numeric strings 50% shorter.

Structural Anatomy

Every Code 128 symbol follows a strict physical structure defined by ISO/IEC 15417:

1. Start Character: Identifies which subset (A, B, or C) the barcode begins with.
2. Data Characters: The encoded information payload.
3. Check Digit: Calculated using the Modulo 103 algorithm. It ensures data integrity but is not part of the human-readable text.
4. Stop Character: A unique pattern signaling the end of the data.
5. Quiet Zone: A mandatory blank margin (at least 10 times the width of the narrowest bar).