Code 128 barcode generator

Code 128 is the general-purpose linear barcode. It covers the whole 128-character ASCII set — upper and lower case letters, digits, punctuation and the control characters — which is why almost every shipping label, warehouse bin card and asset tag you will ever print uses it. Unlike EAN or UPC, nobody allocates Code 128 numbers: you decide what the symbol says, and it means whatever your own system decides it means.

Its density comes from three interchangeable character sets. Code set A (Code 128A) carries upper case and control characters, code set B (Code 128B) upper and lower case and punctuation, and code set C (Code 128C) packs two digits into a single symbol character. A good encoder switches between them part-way through a symbol, so a long run of digits ends up about half the width it would need in any other linear format. bwip-js, which draws the preview above, chooses the code sets for you.

The variant you meet in logistics is GS1-128, once called UCC/EAN-128: the same symbology with an FNC1 character in front and GS1 Application Identifiers structuring the payload, so a scanner can tell a batch number from an expiry date. If a carrier has sent you an SSCC label specification, GS1-128 is what they are asking for.

Every Code 128 symbol character has a value from 0 to 106 and is drawn as three bars and three spaces, eleven modules wide in total; only the stop pattern has a fourth bar and is thirteen modules wide. What a value means depends on the code set in force. The start character — Start A, Start B or Start C — selects the first set, the Code A, Code B and Code C characters switch sets part-way through, and Shift changes between A and B for the next character only.

Code 128A covers upper-case letters, digits, punctuation and the ASCII control characters NUL to US. Code 128B covers the printable range from space to tilde, including lower case, and is where most text starts. Code 128C encodes the pairs 00 to 99 as single characters, which is why a long even-length number comes out at half the width. FNC1 directly after the start character marks a GS1-128 symbol; FNC2, FNC3 and FNC4 are for message appending, reader programming and extended ASCII, and are rarely needed.

The check character is the start value plus each data value multiplied by its position, taken modulo 103. You never have to work it out, or choose a code set: this generator does both. The table is for reading a symbol by hand, checking another encoder, or building a font.

Code 128 is decoded by measuring the ratio between wide and narrow bars, so anything that thickens or thins the bars unevenly breaks it. Print black on white, export SVG when the label goes to a professional printer, and never stretch the symbol horizontally to make it fill a box in your layout.

Keep the quiet zone. Ten narrow-bar widths of blank space on each side is part of the symbology rather than decoration, and cropping it is the single most common reason a technically correct label refuses to read.