Code 128 barcode generator

What this tool does, exactly

Code 128 is the barcode for everything that is yours: asset tags, shelf locations, order and batch numbers, membership cards. It carries letters, digits and punctuation at any length, needs no number issued by anyone, and every ordinary scanner reads it. Type the text and the symbol is drawn with its check character and quiet zone in place.

What it holds

All 128 ASCII characters: capitals, lower case, digits, spaces and ordinary punctuation, with no fixed length. The symbol grows wider with every character, so a fifty-character code is a long strip. Anything outside ASCII — an accented letter, an em dash — is refused rather than silently swapped, because a scanner would read something different from what you meant.

The three code sets

Code 128 switches internally between set A (capitals and control characters), set B (upper and lower case) and set C (pairs of digits packed into one symbol). The encoder here starts in whichever set makes the symbol shortest and switches mid-stream when that saves space, which is why a run of digits comes out at roughly half the width of the same digits in a text code. None of this is anything you have to choose.

The check character

Every Code 128 symbol ends in a check character computed as a weighted sum of everything before it, modulo 103. A scanner recomputes it on every read and rejects a symbol that does not match, so a smudged bar produces no read rather than a wrong one. It is drawn for you; there is no version of Code 128 without it.

Printing

Code 128 has no nominal size; what matters is the narrowest bar, which should be at least 0.19 mm for a handheld scanner and more for a fixed one, and the quiet zone of ten modules either side, which is drawn into the file. Scale proportionally only, print black on white, and take the SVG for labels that go through a thermal printer.