The Universal Product Code (UPC) is a barcode symbology (i.e., a specific type of barcode) that is widely used in the United States, Canada, the United Kingdom, Australia, New Zealand, and in other countries for tracking trade items in stores.

The most common form, UPC-A, consists of 12 numerical digits, which are uniquely assigned to each trade item. Along with the related EAN barcode, the UPC is the barcode mainly used for scanning of trade items at the point of sale, per GS1 specifications. UPC data structures are a component of GTINs (Global Trade Item Numbers). All of these data structures follow the global GS1 specification which is based on international standards. Some retailers (clothing, furniture) do not use the GS1 System (other bar code symbologies, other article number systems). Other retailers use the EAN/UPC bar code symbology but without using a GTIN (for products brands sold at such retailers only).

Composition

Each UPC-A barcode consists of a scannable strip of black bars and white spaces, above a sequence of 12 numerical digits. No letters, characters, or other content of any kind may appear on a standard UPC-A barcode. The digits and bars maintain a one-to-one correspondence - in other words, there is only one way to represent each 12-digit number visually, and there is only one way to represent each visual barcode numerically.

Formatting

UPC-A barcodes can be printed at various densities to accommodate a variety of printing and scanning processes. The significant dimensional parameter is called x-dimension, the ideal width of single module element. A single x-dimension must be used uniformly within a given UPC-A barcode. The width of each bar and space is determined by multiplying the x-dimension by the module width of each bar or space (1, 2, 3, or 4 units). Visually, a grouping of two or more adjacent bars appear as a single wide bar, while a grouping of two or more adjacent spaces appear as a single wide space. Since the guard bars each include two bars, and each of the 12 digits of the UPC-A barcode consists of two (wide) bars and two (wide) spaces, all UPC-A barcodes consist of exactly (3 × 2) + (12 × 2) = 30 (wide) bars, of which 24 represent numerical digits and 6 represent guard bars.

The x-dimension for the UPC-A at the nominal size is 0.33 mm (0.013 in.). Nominal symbol height for UPC-A is 25.9 mm (1.02 in.). In UPC-A the dark bars forming the Start, Middle, and End guard bars are extended downwards by 5 times x-dimension, with a resulting nominal symbol of height of 27.55 mm (1.08 in.) This also applies to the bars of the first and the last symbol characters of UPC-A symbol. UPC-A can be reduced or magnified anywhere from 80% to 200%.

A quiet zone, with a width of at least 9 times the x-dimension, must be present on each side of the scannable area of the UPC-A barcode. UPC-E requires 9 X-dimension units on the left side and 7 on the right. For a GTIN-12 number encoded in a UPC-A barcode symbol, the first and last digits are always placed outside the symbol to indicate the quiet zones that are necessary for barcode scanners to work properly.

Numbering

UPC-A and UPC-E each provide a theoretical maximum of 1 trillion (10^12) unique barcodes, though in practice the number of barcodes is limited by the standards used to create them. For instance, the last digit is the check digit and therefore can only be one correct value for UPC-A. This gives only 100,000,000,000 (10^11) possibilities.

UPC-A: (10 possible values per digit ^ 6 left digits) × (10 possible values per digit ^ 6 right digits) = 1,000,000 × 1,000,000 = 1,000,000,000,000.
UPC-E: 10 possible values per digit × 2 possible parities per digit = 20 permutations per digit ^ 6 digits = 64,000,000.

Number system

The first digit indicates the number system to be used for the subsequent digits. The following number system digits and their numbering schemes are:

• 0, 1, 6, 7, 8: For most products. The LLLLL digits are the manufacturer code, and the RRRRR digits are the product code.
• 2: Reserved for local use (store/warehouse), for items sold by variable weight. Variable-weight items, such as meats and fresh fruits and vegetables, are assigned a UPC by the store, if they are packaged there. In this case, the LLLLL is the item number, and the RRRRR is either the weight or the price, with the first R determining which.
• 3: Drugs by National Drug Code number. Pharmaceuticals in the U.S. have the remainder of the UPC as their National Drug Code (NDC) number; though usually only over-the-counter drugs are scanned at point-of-sale, NDC-based UPCs are used on prescription drug packages and surgical products and, in this case, are commonly called UPN Codes.
• 4: Reserved for local use (store/warehouse), often for loyalty cards or store coupons.
• 5, 9: Coupons: The manufacturer code is the LLLLL, the first 3 RRR are a family code (set by manufacturer), and the next 2 RR are a coupon code. This 2-digit code determines the amount of the discount, according to a table set by the GS1 US, with the final R being the check digit. These coupons can be doubled or tripled.

Check digits

In the UPC-A system, the check digit is calculated as follows:

1. Add the digits in the odd-numbered positions (first, third, fifth, etc.) together and multiply by three.
2. Add the digits in the even-numbered positions (second, fourth, sixth, etc.) to the result.
3. Find the result modulo 10 (i.e. the remainder when divided by 10.. 10 goes into 58 5 times with 8 leftover).
4. If the result is not zero, subtract the result from ten.

For example, in a UPC-A barcode "03600029145x" where x is the unknown check digit, x can be calculated by:

1. Adding the odd-numbered digits (0 + 6 + 0 + 2 + 1 + 5 = 14)
2. Multiplying by three (14 × 3 = 42)
3. Adding the even-numbered digits (42 + (3 + 0 + 0 + 9 + 4) = 58)
4. Calculating modulo ten (58 mod 10 = 8)
5. Subtracting from ten (10 − 8 = 2)

The check digit is thus 2.

This should not be confused with the numeral "X" which stands for a value of 10 in modulo 11, commonly seen in the ISBN check digit.

Source: Wikipedia