Barcode Secrets

On this page we reveal the secrets behind the Universal Product Code, (or Barcode). Although there are many formats for these codes, the Universal Product Code stands as an example for the information we have on this page. It has always been rumoured that if we changed a few bars here and there we may be able to fool the supermarket scanners into reading the code as a lower price. How feasible is this? Not very. For one, the price of the item is not stored in the barcode. Only the product name and manufacturers code is listed there, and the price is called up from the supermarkets central computer database. When we thought about it, we realised that the product manufacturer wasn't going to change his packaging every time the supermarket wanted to special their products or increase the prices, so it stands to reason that the price is not included in the UPC. The scanners that are in use today at even the smallest store can detect any attempt at deception right away. Though not necessarily detecting foul play, the scanner simply interprets the code as being invalid or that the product is not listed. How does it know? There is a secret digit called a 'Check Digit' tacked on to the end of the UPC. This code is derived mathematically from the the information in the rest of the code. If this check number does not match with the rest of the code then an error is returned. As we shall see, even knowing this secret does not help the cheating purchaser.

The check digit system was developed by the Uniform Product Code Council, a manufacturers association based in Dayton, Ohio. Every other manufacturer calculates the check digit in the same way.

Usually, the UPC looks very much like the following:

There are shorter and longer forms of the UPC, however the most common format is shown above. There is a number to the left of the first bars, then a group of five, or six numbers, more bars, another group of five or six numbers and, occasionally, another number to the right of the last bar. A shorter format has only six digits in the code. This is used for small items where placing the full code on the packaging is impractical. We will go through all the details and see how they arrive at the check digit figure.

The first lines we see are two narrow parallel bars. These are mirrored in the centre of the UPC and again at the very end. Called 'Guard Bars' they serve no other function than to tell the scanner where to start reading.

The next bars denote the type of product, 0 for groceries, 2 for variable weight products, 3 for pharmaceuticals and health related items, 5 for discount 'coupon' items and so on.

The first set of 5 or 6 digits denote the manufacturer. The second set denote the product and size. All companies assign their own codes and if 12345 means a can of Tuna from one company it wont mean the same for another manufacturer, both sets of numbers being taken into account when the code is read by the scanner.

The bar pattern can vary considerably in size, from a minimum of 0.819 x 1.175 inches to 2.040 x 2.938 inches maximum. The bars can be any colour except red, although most modern scanners will now read red codes. The area of the UPC symbol allotted to each digit is composed of seven equal 'modules', each of which may be dark or light. For example, the symbol for zero is three light modules (the space), two dark modules (the thick bar), one light module (the narrow space) and one dark module (the narrow bar). This represents 0001101, where the zeros are light modules and the ones are dark modules. So the barcode for zero is: wide space, bar, narrow space, thin bar. (If this sounds complicated, don't worry. The numbers are always printed underneath the code, and, as scanners don't read numbers, they are there only to allow manual input in case of scanner failure.) It gets worse. The code for zero above is only true if the zero is on the left side of the central guard bars. The code for zero when it is to the right of the guard bars is 1110010. Why? So the scanner can read the code from any direction. The check digit is encoded according to the right hand column. How?

The following formula is used to calculate the check digit from a barcode.

Take the digits of the code (the example above would be 1 23456 78901) and group them according to their sequence in the code. Put the odd digits in a line at the top and the even ones in a line below.

1 3 5 7 9 1

2 4 6 8 0

Then add each sequence of digits,

1 + 3 + 5 + 7 + 9 + 1 = 26

2 + 4 + 6 + 8 + 0 = 20

Multiply the sum of the odd sequence by 3,

26 X 3 = 78

Add the result to the sum of the even sequence,

78 + 20 = 98

Subtract this result from the next highest multiple of 10, in this case 100,

100 - 98 = 2

The remainder, 2, is our correct check digit.

As you can see, changing any digit above will alter the check digit and therefore result in a scanner error. There is no case where a customer could change the UPC with a marker pen to that of a lower cost item. Chances are that any change would result in an unassigned code error. If by chance the code was valid, the odds would be even as to whether it would change it to a lower or a higher priced item.

Thanks for stopping by our barcode area. If you have any further information that we could use then please mail us here and let us in on the secret.  

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