The "UPC-A bar code" is by far the most common and well-known symbology, at least in the United States.
An UPC-A bar code is the bar code you will find on virtually every consumer good on the shelves of your local supermarket,
as well as books, magazines, and newspapers.

It is commonly called simply a "UPC bar code" or "UPC Symbol."

UPC-A barcode can encode numeric data from 0 through 9.

An UPC-A bar code is divided into four areas:

- The number system
- The manufacturer code
- the product code
- the check digit

UPC-A encodes 11 digits of numeric (0 through 9) message data along with a trailing check digit, for a total of 12 digits of bar code data.

The UPC-A number system is a single digit which identifies the product type, if you will, that the following symbol represents.

The table below indicates what each number system identifies.

Number System | Description |

0 | Regular UPC codes |

1 | Reserved |

2 | Weight items marked at the store |

3 | National Drug/Health-related code |

4 | No format restrictions, in-store use on non-food items |

5 | Coupons |

6 | Reserved |

7 | Regular UPC codes |

8 | Reserved |

9 | Reserved |

The UPC-A checksum digit is based on a modulo 10 calculation based on the weighted sum of the values of each of the digits
in the number system, manufacturer code, and product code.

The steps for calculating the check digit are as follows:

- Consider the right-most digit of the message to be in an "odd" position, and assign odd/even to each character moving from right to left.
- Sum the digits in all odd positions, and multiply the result by 3.
- Sum the digits in all even positions.
- Sum the totals calculated in steps 2 and 3.
- The check digit is the number which, when added to the totals calculated in step 4, result in a number evenly divisible by 10.
- If the sum calculated in step 4 is evenly disivisible by 10, the check digit is "0" (not 10).