Find the barcode number of an item by identifying the 12-digit number located beneath the UPC image, which is the vertical bar graphic attached to the product or incorporated into the package design. The first 11 digits are the actual product code, while the 12th number is the check digit.
Barcode numbers are often typed directly into a point-of-sale system to determine product price, rather than scanning the graphic. These numbers are numeric representations of the vertical bars above them. Optical scanners read these bars and determine the width and "quiet zones" of the graphic to arrive at a number sequence.
The back-end computer uses the check digit to verify that a UPC-A barcode number has been interpreted correctly. A series of mathematical calculations with the first 11 digits returns the final digit if the barcode number is valid.