If users are unable to print item labels, the item barcode may be invalid. A barcode is invalid when its check digit is incorrect. A check digit is a number appended to the data contained in a barcode. A Check digit is used to assure accuracy; to "check" for errors. It typically consists of a single digit computed from the other digits in the data.

To check if a barcode is valid, consider the following:

- For 12 digit bar codes (UPCA types of bar codes)
- Sample bar code is 134688257347

- Add the digits in the odd-numbered positions (first, third, fifth, etc.) together and multiply the total by three.

■ 1+4+8+2+7+4=26

- Multiply what you got on step 1 by 3

■ 26 * 3 = 78

- Add the digits in the even-numbered positions (second, fourth, sixth, etc.).

■ 3+6+8+5+3=25

- Add the two results together.

■ 78 + 25 = 103

- Now what single digit number makes the total a multiple of 10? That's the check digit.

■ 103, to make it a multiple of 10, it needs 7

■ 7 is the check digit that must be the last digit of the bar code

- For 13 digit bar codes (EAN type)
- Sample bar code 1346882573478

- Starting from the second position, add all the digits in an even-numbered positions (second, fourth, sixth) together.

■ 3+6+8+5+3+7=32

- Multiply what you got on step 1 by 3

■ 32*3=96

- Add all the digits in an the odd-numbered positions together

■ 1+4+8+2+7+4=26

- Add the two results together

■ 96+26=122

■ 122, to make it a multiple of 10, it needs 8

■ 8 is the check digit that must be the last digit of the bar code