Code 128 Barcode Generator

Quick answers

What is a Code 128 barcode used for?

Internal labelling: SKUs, warehouse shelves and bins, shipping cartons, asset tags, ID badges and sample or test labels — anywhere an organisation invents its own references and needs machines to read them reliably. It is not the symbology used for retail checkout barcodes, which are UPC-A and EAN-13 carrying licensed GTINs.

Code 128 earned its place by density and range: it encodes all 128 ASCII characters and, in Code C, two digits per symbol, so long numeric references stay short on the label. GS1-128 — the logistics variant with its own application-identifier rules — is built on Code 128, which is why the same bars show up on pallets and cartons throughout the supply chain.

How does the Code 128 checksum work?

Add the start code’s value to each following symbol’s value multiplied by its position (first symbol × 1, second × 2, and so on), then divide by 103. The remainder is the checksum symbol’s value, and it is printed as part of the barcode, just before the stop pattern.

For HELLO in Code B: start 104, then H 40×1, E 37×2, L 44×3, L 44×4, O 47×5 — a total of 761, and 761 mod 103 is 40, so the checksum symbol is value 40. Scanners repeat this arithmetic on every read and refuse the barcode if the remainder disagrees, which is why a damaged or misprinted Code 128 usually fails loudly instead of reading as a different, wrong code.

Which code set should I choose — A, B or C?

You do not have to: this generator chooses automatically. Code C for digit runs (two digits per symbol), Code B for ordinary mixed text with lower-case letters, Code A only when the text contains control characters. The symbol-by-symbol breakdown under your barcode shows exactly which sets were used and where it switched.

Manual choice matters mainly when another system dictates the encoding. Left automatic, the rule that shapes the result is simple: four or more digits in a row are worth a trip into Code C, and anything containing lower-case letters or characters above ASCII 95 must pass through Code B. Pure numbers of an even length are the ideal case — start in C and never switch.

How to use

1. Type the text to encode — a SKU, an internal reference, a short label string. ASCII characters only.
2. Pick a bar height for your label stock, then look at the preview and the symbol breakdown under it.
3. Download the PNG for documents and label software, or the SVG when the barcode will be scaled or print-produced.
4. Print one label and scan it with the actual scanner or phone app your team uses before printing a run — quiet zones and print quality decide real-world reads.

About this tool

The set switching is the part worth understanding, because it is where naive generators waste label space. Code C reads digits in pairs, so a 12-digit internal reference costs six symbols instead of twelve; Code B covers lower-case letters that Code A cannot reach; Code A exists for the top row of ASCII and control characters. This generator picks the start set from your text, switches to Code C for digit runs of four or more, and latches back afterwards. The breakdown under the barcode lists every symbol it used, so you can audit the encoding instead of trusting it.

One boundary matters more than any setting on this page: a barcode is a font for numbers, not a licence for them. This generator is for labels that live inside your own operation. The barcode on a product sold in shops is a different thing — a GTIN you license from GS1, usually printed as UPC-A or EAN-13 — and no generator, including this one, can issue you one. There is a companion validator on this site that checks the check digits of the codes you already have.

Everything runs in your browser. The text you encode is never uploaded, sent to a server or stored, and the barcode is drawn locally as vector shapes.

How it works

Encoding (ISO/IEC 15417): the text is converted to codeword values 0–102 in code sets A, B or C (in B and A, value = ASCII − 32 for printable characters; in C, each pair of digits is one value 0–99), preceded by a start code (103, 104 or 105) and followed by a checksum and the stop pattern (106). Code-set switches are codewords 99 (to C), 100 (to B) and 101 (to A) in the sets where they apply.

Checksum: checksum value = (start value + Σ symbol value × position) mod 103, positions counting from 1 for the first symbol after the start code. Each codeword is drawn as three bars and three spaces totalling 11 modules (the stop pattern takes 13), flanked by quiet zones of at least 10 modules that this generator always includes.

Worked examples

A warehouse SKU: WH-2049-BLUE: Mixed letters, digits and hyphens — the generator stays in Code B, where the four-digit run is short enough that a trip into Code C would cost as much in switch symbols as it saves.

A long numeric reference: 88412093745516: Fourteen digits, an even count, so the whole string rides in Code C: start C, seven pair symbols (88, 41, 20, 93, 74, 55, 16), checksum, stop — half the width of the same number in Code B.

Checking the checksum by hand: HELLO: Start B (104) + H 40×1 + E 37×2 + L 44×3 + L 44×4 + O 47×5 = 761; 761 mod 103 = 40. The breakdown under the generated barcode shows checksum value 40 — the same number your scanner will compute when it reads the label.

The three code sets, briefly

Frequently asked questions

More free tools