UPCBay

What Is a Check Digit?

The check digit is the last digit in a UPC or EAN barcode. It's calculated mathematically from the preceding digits and exists purely to catch errors. When a scanner reads a barcode, it recalculates the check digit from the other numbers and compares. If they match, the scan is valid. If they don't — misprint, smudge, or typo — the scanner rejects the read.

Real example

Take UPC 01234567890?. The first 11 digits are the actual product code. The 12th digit is the check digit, calculated using a specific formula: multiply alternating digits by 1 and 3, add them all up, and the check digit is whatever number makes the total a multiple of 10. For 01234567890, the check digit is 5 — giving you 012345678905.

How a Check Digit is used in e-commerce

  • Every UPC-A, EAN-13, and EAN-8 barcode includes a check digit as the final number.
  • Amazon's GTIN validation checks the check digit — a barcode with an incorrect check digit will fail listing creation.
  • Barcode printing software and label generators calculate the check digit automatically.
  • The GS1 check digit calculator (gs1.org) can verify whether any barcode number is structurally valid.

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Common questions

Related terms

UPC codeEAN codeGTIN