Make a UPC label
Prints on 30-per-sheet letter labels (Avery 5160 size). Many items at once? Use the label maker.
How the check digit works
The last digit of a UPC is calculated from the other eleven, so a scanner can tell when a code was misread. Starting from the right of the first eleven digits, multiply every other digit by 3, add everything up, and the check digit is whatever brings the total to the next multiple of 10. This tool does that for you; a wrong final digit shows the correct one.
UPC-A or EAN-13?
Use the code printed on the product. UPC-A (12 digits) is the standard in the US and Canada; EAN-13 (13 digits) everywhere else. A UPC-A is the same as an EAN-13 with a 0 in front, and scanners read both.
Where do UPC numbers come from?
To sell in stores that require them, a company licenses a prefix from GS1, the standards body that issues UPC and EAN numbers, and numbers its products from there. For your own shelves, bins and parts you don't need UPCs: use your SKU with a Code 128 barcode in the label maker.