Code 128 Barcode Generator

Character set, subsets, and length rules

Code 128 encodes data using three interchangeable character subsets. Subset A covers uppercase letters, digits, punctuation, and ASCII control codes. Subset B covers upper- and lowercase letters, digits, and punctuation — the subset used for most human-readable labels. Subset C is numeric-only, but it packs two digits into a single symbol character instead of one, which roughly halves the length of long numeric strings. A capable encoder switches between subsets automatically, mid-barcode, to keep the result as compact as possible for whatever you type.

Length limits

There's no official maximum length written into the specification — the practical limit comes from your label size and the scanner's depth of field, not from the symbology itself. Strings longer than about 20-30 characters start to get visibly wide, so leave enough label real estate for whatever you're encoding.

Check digit

Code 128 always includes a built-in checksum — a modulo-103 check character — but you never see or manage it yourself. It's calculated automatically as part of encoding and isn't one of the digits you type. That's different from GTIN-family formats like EAN-13 or UPC-A, where the check digit is a visible digit printed under the bars.

Where Code 128 is used

Code 128 turns up anywhere a barcode needs to carry arbitrary text rather than a fixed-format retail number: shipping and logistics labels (it's the base symbology behind GS1-128), inventory and asset tags, patient wristbands, event and conference badges, library cards, and internal SKU or lot-number labels. It's rarely the format read at a retail checkout scanner, where EAN-13 and UPC-A are what's expected — use Code 128 for anything that isn't a shelf product, and a dedicated retail symbology for anything that is.

Sizing and printing guidance

Code 128's minimum quiet zone — the clear, unprinted margin on either side of the bars — is 10 times the narrow bar width, known as the x-dimension. At a common x-dimension of 0.25 mm, that works out to roughly a 2.5 mm clear margin on each side. Crowding that margin with text, a border, or another barcode is one of the most common reasons a barcode that looks fine on screen fails to scan once printed.

Printing guidance

For labels a handheld or fixed scanner will read reliably, print at 300 dpi or higher — thermal-transfer and most laser printers handle this without issue, though cheap inkjet output can blur narrow bars enough to cause misreads. Avoid scaling a barcode down below about 80% of its designed size, and keep it pure black bars on a white or light background; low-contrast combinations (light gray on white, red on white) fail scanners even when they look fine to the eye.