# Check Digit Calculation

The Check Digit is the last, rightmost digit of the number and is based on a mathematical algorithm called the Modulo 10. The check digit is a key control in the accuracy of the GTIN.  It is a computer check to make sure the bar code is correctly composed.  When a scanner reads a bar code symbol, it will perform the same calculation and verify that the two results match.  In this way the scanner knows it has read all the digits of the number correctly.

There are 5 steps involved in calculating the check digit manually.

1. Taking the sample GTIN 539 123 456 789, start with the digit on the right side of the number (i.e. the 9) and sum all the alternate digits, reading from right to left: 9 + 7 + 5 + 3 + 1 + 3 = 28
2. Multiply the result of Step 1 by 3: 28 x 3 = 84
3. Again from right to left sum all the other remaining digit values: 8 + 6 + 4 + 2 + 9 + 5 = 34
4. Add the result of Step 2 to the result of Step 3: 84+34 = 118
5. The Check Digit is the smallest number, which when added to the result of Step 4, produces a multiple of 10: 118 + x = 120. Answer is 2.

In this example, the next multiple of ten after 118 is 120. Therefore the Check Digit is the difference between 120 and 118, which is 2.

The complete number is 5 3 9 1 2 3 4 5 6 7 8 9 2

If you are generating many GTINs you may wish to use the online check digit calculator which is available on this website.

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