Barcode Generator (Code 128, EAN-13, UPC-A)

UPC-A and EAN-13 are retail symbologies: a point-of-sale scanner expects a number that GS1 has assigned to your company. If you just need to label bins, inventory or equipment, Code 128 is the better choice because it accepts letters and punctuation and is compact for long strings.

UPC-A and EAN-13 use the GS1 mod-10 check digit. Starting with the rightmost data digit, multiply digits alternately by 3 and 1, add the products, and take the amount needed to reach the next multiple of 10. For 400638133393 the weighted sum is 89, so the check digit is 1 and the full EAN-13 is 4006381333931. Type 11 UPC or 12 EAN digits and the generator appends it; type the full number and it tells you if the last digit is wrong.

Code 128 uses a mod-103 checksum that is encoded as a bar pattern but never printed. Each symbol value is multiplied by its position (the start code counts once), the products are summed, and the remainder after dividing by 103 becomes the check symbol. Strings made only of an even number of digits are packed two digits per symbol with subset C, which roughly halves the barcode width; other text uses subset B.

Every EAN-13 or UPC-A symbol is 95 modules wide: a 101 start guard, six left digits of 7 modules, a 01010 center guard, six right digits and a 101 end guard. The first EAN digit is not drawn directly; it selects the odd/even parity pattern of the left half.