Code 128

What is a Code 128 barcode?

Code 128 is a high-density linear barcode symbology capable of encoding all 128 ASCII characters. Introduced in 1981 by Computer Identics Corporation, it is widely used in logistics, shipping labels (FedEx, UPS, USPS), and GS1-128 supply chain barcodes. Unlike EAN/UPC barcodes, Code 128 has no fixed length — it can encode as many characters as needed, making it extremely versatile for variable-data applications.

Code subsets explained (A, B, C)

Code 128 uses three internal code sets:

• Code A: ASCII 00–95 (control characters + uppercase).
• Code B: ASCII 32–127 (full printable ASCII).
• Code C: Pairs of digits (00–99) encoded at double density — ideal for long numeric strings like SSCC shipping container codes.

A single barcode can switch between subsets mid-symbol using special switch codewords, allowing optimal compression.

Check character algorithm

Code 128 uses a weighted modulo-103 check character (not Modulo 10):

1. Start with the start-character value (A=103, B=104, C=105).
2. For each subsequent data codeword, multiply its value by its position index (1, 2, 3…).
3. Sum the start value + all weighted products.
4. Check character = total mod 103.

This is calculated automatically by all barcode generators and verified by every Code 128 scanner.

Code 128 vs. Code 39 vs. QR Code

Code 128 is denser than Code 39 (encodes the same data in ~30% less space) and supports the full ASCII set, whereas Code 39 only covers 43 characters natively. For 2D applications, QR Code stores more data in less physical space but requires a camera-capable scanner. Code 128 is preferred when linear scanners are in use and data includes lowercase letters or special characters.

When to use Code 128

Code 128 is the right choice for: shipping and logistics labels, GS1-128 (formerly UCC/EAN-128) supply-chain data, healthcare barcodes (HBC), warehouse rack labels, and any application requiring full ASCII encoding in a linear barcode.

Printing requirements

Minimum X-dimension (narrowest bar): 0.19 mm (7.5 mil) for handheld scanners; 0.25 mm for fixed scanners. Quiet zones: 10× X-dimension each side. No mandatory height, but GS1-128 requires minimum 32 mm bar height. Always include human-readable text below for manual entry fallback.

Common scanning failures

Excessively narrow X-dimensions (below printer resolution), missing quiet zones, and incorrect start/stop characters are the top Code 128 failure modes. When printing GS1-128 FNC1 application identifiers, ensure your generator inserts the FNC1 character correctly — a missing FNC1 makes the barcode invalid for GS1 supply chain use.

Comparison