How the GS1 Mod-10 Algorithm Determines Your Barcode's Check Digit

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Every GTIN barcode — including UPC-A, EAN-13, and GTIN-14 — ends with a check digit calculated using the GS1 Mod-10 algorithm, not assigned randomly. The algorithm multiplies each digit by alternating weights of 3 and 1 from right to left, sums the products, and derives the check digit as the value needed to reach the next multiple of 10. An incorrect check digit is the most common reason product listings get rejected on platforms like Amazon, Google Merchant Center, and Shopify. A frequent source of errors is Excel automatically stripping leading zeros from barcodes, which changes the digit count and invalidates the code. Developers can implement the validation in a few lines of Python, and bulk feed validation is also possible via deterministic JSON APIs.

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