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. Alo

ng 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
__R__RRRR 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:

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

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

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

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