Code 128 Barcode Generator

How to use the code 128 barcode generator

1. Type or paste the ASCII text or numbers you want to encode into the Data field.
2. Adjust the Bar height slider if you need taller or shorter bars.
3. Toggle Show human-readable text to include or hide the label below the barcode.
4. Choose a Background of white or transparent depending on where you will place the image.
5. Click Generate to render the Code 128 barcode as an SVG you can copy or download.

How Code 128 encoding works

Code 128 encodes each character as a pattern of three bars and three spaces spanning exactly 11 modules. Three code sets cover different character ranges: Set A handles control characters and uppercase, Set B covers printable ASCII (space through tilde), and Set C encodes pairs of digits for maximum density.

A valid symbol always starts with a start character that selects the initial code set, followed by the data characters, a check character, and a stop pattern that is 13 modules wide. The check character is computed using a weighted sum modulo 103. Each data position is multiplied by its one-based index, added to the start code value, and divided by 103; the remainder is the check value.

For example, encoding the letter "A" in Set B: Start B has value 104. The character "A" has value 33 at position 1. The weighted sum is 104 + (33 x 1) = 137. Dividing 137 by 103 gives a remainder of 34, so the check character value is 34.

Automatic switching between code sets B and C

This code 128 barcode generator uses an optimization algorithm that scans the input string for runs of digits. When four or more consecutive digits appear (and the count is even), the encoder switches to Set C, which packs two digits into a single 11-module symbol instead of using two separate symbols in Set B. For odd-length digit runs of five or more, it encodes the first digit in Set B and the remaining even-length portion in Set C.

Switching costs one symbol for the code change command, so it only saves space when the digit run is long enough to offset that cost. Shorter digit sequences stay in Set B to avoid unnecessary switching overhead. The resulting code set sequence is displayed in the output rows so you can verify how the data was encoded.

Module widths and quiet zones

Every Code 128 symbol has a predictable width. Start, each data character, and the check character are 11 modules wide. The stop pattern is 13 modules. The specification requires a quiet zone of at least 10 modules on both the left and right sides, free of any marks or print.

The total module count shown in the results includes these quiet zones. For instance, encoding "A" produces: 10 (left quiet) + 11 (start) + 11 (data) + 11 (check) + 13 (stop) + 10 (right quiet) = 66 modules. In the generated SVG each module maps to a coordinate unit, making the barcode resolution-independent.

Common use cases

Code 128 is widely used in shipping labels, inventory management, and healthcare wristbands because it supports the full ASCII set while remaining compact. Logistics carriers like FedEx and UPS rely on it for tracking numbers. Unlike Code 39, which encodes each character in 12 or more modules without a dedicated digit-packing mode, Code 128 achieves higher density, especially for numeric-heavy data.

If you need a general-purpose tool for multiple symbologies, the barcode generator page covers several formats. For two-dimensional codes, consider the Data Matrix generator or QR code generator instead.

Reference table of character values

The table below shows selected character values used in Set B encoding. These values feed directly into the modulo 103 check calculation.

In Set C, values 0 through 99 represent digit pairs "00" through "99". Start B is 104, Start C is 105, Code C switch is 99, and Code B switch is 100.

Worked examples

Each result below was produced by the same code the tool runs, so you can check your own numbers against them.