Most items manufactured for sale through retail outlets bear a number and a bar code symbol like the one shown below. The whole number is made up of two sets of digits and a check digit.

GS1 Company Prefix

This is allocated by GS1 INDIA. It enables unique identification for an organisation  throughout the world.

This prefix simply provides the initial seven,  eight or nine digits of the 13 digits barcode number.

Item Id

The GS1 subscriber uses these digits to allocate unique number for each product variant.

Check Digit

A calculation using the preceding twelve digits is used to produce this number. Every time a computer decodes the bar symbol it repeats the calculation and checks its answer against the check digit to verify that the symbol has been correctly scanned and decoded.

Primary identification numbers may also be eight and 14 digits, depending on the item they are used for.

How to calculate the Check Digit

A check digit is the result of a mathematical calculation performed on the preceding digits in the number. It is used during scanning to verify that the barcode was correctly read.

This calculation, called the Modulo 10 algorithm, is the same for all GS1 primary identification numbers.

An example calculating the check digit

8 9 0 4 0 0 0 2 1 0 0 3 C (check digit position)

Perform the following calculation using the GTIN - 13 numbers from the example above.

1. Starting with the first number on the right, add all the alternate numbers 3 + 0 + 2 + 0 + 4 + 9 = 18
2. Multiply the result by three.
   18 x 3 = 54
3. Starting with the second number on the right, add all alternate numbers.
   0 + 1 + 0 + 0 + 0 + 8 = 9
4. Add the results of steps two and three.
   54 + 9 = 63
5. Add the number needed to bring the total to the next multiple of ten.
   63 + 7 = 70

The number which you need to add to make it the next multiple of 10 is the check digit. In our example the check digit is 7.

The complete number is:

     890 4000 21003 7