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The UPC (now officially EAN.UCC-12) encodes twelve digits as SLLLLLLMRRRRRRE, where S (start) and E (end) are the bit pattern 101, M (middle) is the bit pattern 01010 (called guard bars), and each L (left) and R (right) are digits, seven bits long each. This is a total of 95 bits. The bit pattern for each numeral is designed to be as little like the others as possible, and to have no more than four 1s or 0s in order. Both are for reliability in scanning.

The UPC is only numerals, with no letterAn alphabet is a complete standardized set of letters—basic written symbols—each of which roughly represents or represented historically a phoneme of a spoken language. This as distinguished from other writing systems such as ideograms, in which symbols rs or other characterPunctuation marks are written symbols that do not correspond to either phonemes (sounds) of a spoken language nor to lexemes (words and phrases) of a written language, but which serve to organize or clarify written language. See orthography. The rules ofs. The first L digit is 0 for ordinary items, 3 for pharmaceuticals, 2 for random- weightFor the 1994 album by the group Rollins Band, see Weight (album). Weight is the force exerted upon an object by virtue of its position in a gravitational field. In a constant gravitational field, such as the Earth's, this force is proportional to the obje items, and 5 for couponA coupon is a ticket or document that can be exchanged for a financial discount on a product. Customarily, coupons are issued by retail stores as a part of sales promotions. In recent years they are often widely distributed, through mail, magazines, and ns (though stores often ignore this and use 000000 or 999999). The rest of L is the manufacturer code. The first five R digits are the product code assigned by the manufacturer. The last digit R is a check digitIn computers and telecommunication, a check digit is a simple form of redundancy check used for error detection. It consists of a single digit computed from the other digits in the message. The final digit of a Universal Product Code is a check digit comp, so that errors in scanning or manual entry can be detected. In the UPC-A system, the check digit is calculated as follows:

1. Add the odd-numbered digits (first, third, fifth, etc.) together and multiply by three.
2. Add the even-numbered digits (second, fourth, sixth, etc.) to the result.
3. Subtract the result from the next-higher multiple of ten. The answer is the check digit.

For instance, a UPC-A barcode "03600029145X" where X is the 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) and subtracting from the next-higher multiple of ten (60 - 58 = 2). The check digit is thus 2.

Pharmaceuticals in the U.S. have the remainder of the UPC as their National Drug Control (NDC) number. Random-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 _RRRR is either the weight or the price, with the first R determining which. Likewise, coupons are supposed to have the coupon code in LLLLL, the amount to be taken off in _RRRR, and whether that amount is a percent or a literal amount encoded in the first R.