UPC/EAN Check Digit Calculator
A wrong check digit is one of those mistakes that costs you nothing for weeks, then costs you a rejected shipment. I watched a supplier print 3,000 labels with a transposed UPC, and FBA bounced the whole inbound because the barcode didn't scan to a real product. The box sat in limbo while we ate storage and lost a week of sales.
The check digit is the last number in a UPC or EAN. It isn't random — it's computed from all the digits before it using the GS1 mod-10 method. This tool does that math so you can verify a code before it goes to print, or catch a typo the moment a supplier sends you a number.
How the algorithm works
Take the digits before the check digit. Starting from the right, multiply every other digit by 3 and the rest by 1. Add them up. The check digit is whatever makes the total a multiple of 10: (10 minus the remainder) mod 10.
So if your 11-digit UPC body is 03600029145, the tool returns the full 12-digit code 036000291452. For EAN-13 you feed it 12 digits and get 13 back. The result box shows the actual multipliers used, so you can see the math instead of trusting it.
Where this saves you
- Verifying a supplier's barcode before production.
- Catching a fat-finger typo when you type a code by hand.
- Building correct codes for a private-label product that needs its own UPC from GS1.
A barcode that doesn't match the check digit won't validate at Amazon's receive dock. The scanner reads the number, the digit is wrong, and your inventory sits unresponsive until someone fixes it.
Don't trust the label
Printed labels lie. A cheap label printer or a careless supplier will reproduce whatever you send, wrong digit and all. Run every new code through this checker once. It takes three seconds and it's cheaper than a rejected pallet.
If the tool says your input isn't 11 or 12 digits, you've pasted the full code by accident or left a digit out. Strip it to the body and try again.