The last number in a UPC or EAN isn’t random — it’s a check digit that keeps the barcode honest. Here’s what it does.
What the check digit does
The check digit is a built-in error check. A scanner recalculates it from the preceding digits and compares — if they differ, it knows the read was wrong and rejects it.
This is why a single mistyped digit produces an ‘invalid barcode’ error rather than the wrong product.
How it’s calculated
For UPC-A and EAN-13, digits are weighted (alternating x1 and x3), summed, and the check digit is whatever makes the total a multiple of 10.
You don’t need to do this by hand — any barcode generator or validator computes it automatically.
Why it matters for sellers
If you type a number manually and the check digit is wrong, marketplaces reject it. Always copy numbers exactly and validate before listing.
A quick validation step catches transcription errors before they cost you a listing.
Frequently Asked Questions
Where is the check digit?
It’s the very last digit of a UPC or EAN.
Can I pick any check digit?
No — it’s mathematically derived from the other digits.
How do I verify it?
Run the number through a free barcode validator.
Need genuine barcodes? Get ready-to-use UPC & EAN numbers with images at GoodUPC.