The Universal Product Code (UPC) is a barcode symbology (i.e., a specific type of barcode). UPC bar codes were originally created to help grocery stores speed up the checkout process and keep better track of inventory, but the system quickly spread to all other retail products because it was so successful.
The UPC encodes 12 decimal 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, each one represented by a seven-bit code. 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 consecutive 1s or 0s in order.
The first digit L is the prefix. The last digit R is an error correcting check digit, allowing some errors in scanning or manual entry to be detected.
The bar-and-space patterns for each digit 0–9 are as follows:
Digit |
L Pattern |
R Pattern |
0 | 0001101 | 1110010 |
1 | 0011001 | 1100110 |
2 | 0010011 | 1101100 |
3 | 0111101 | 1000010 |
4 | 0100011 | 1011100 |
5 | 0110001 | 1001110 |
6 | 0101111 | 1010000 |
7 | 0111011 | 1000100 |
8 | 0110111 | 1001000 |
9 | 0001011 | 1110100 |
Before the Middle guard bars, a binary 1 is indicated by a bar, while a 0 is indicated by a space. After the Middle guard bars, however, the patterns are optically inverted. In other words, a 1 is now indicated by a space, and a 0 is now indicated by a bar.
Prefixes
- 0, 1, 6, 7, 8, or 9: For most products
- 2: Reserved for local use (store/warehouse), for items sold by variable weight.
- 3: Drugs by National Drug Code number
- 4: Reserved for local use (store/warehouse), often for loyalty cards or store coupons.
- 5: 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
Check digit calculation
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 the result is divided by 10).
- If the result is not zero, subtract the result from ten.
For example, a UPC-A barcode (in this case, a UPC for a box of tissues) “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), calculating modulo 10 (58 mod 10 = 8), subtracting from ten (10 − 8 = 2). The check digit is thus 2.
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