A barcode by definition is simply coded information. The coded information is machine readable – meaning it can be read by a scanner (the machine.) the scanner needs to verify that what it reads is actually what the barcode contains. The Check digit is the way it checks itself.
For example: let us assume that we have a barcode that contains the numbers 1 2 3 4 5.
The check digit will then be: 6
Why 6?
Because the check digit in this case adds all the numbers in the barcode together:
1+2+3+4+5=15 15=1+5=6.
The check digit is normally the right most number in a barcode.
Different barcodes use different ways of calculating the check digit. Most good label design software will perform this calculation automatically.
The GS1 site has a wonderful calculator and they explain in great detail how to calculate check digits for various barcodes using graphics and step by step instructions.
Here is an example of how you calculate a check digit for the GTIN-12 (U.P.C) [taken from the GS1 site]:
GTIN-12 (U.P.C.)
The Check Digit for a GTIN-12 (U.P.C.) ID Number is figured using the standard modulo calculation. Here is how it works:
| Step One: | Suppose you want to find the Check Digit for the GTIN-12 (U.P.C.) Number 61414121022. Set up a table with 12 columns, and put the number 61414121022 into Positions One through Eleven. Position Twelve will be blank because it is reserved for the Check Digit. |
| Step Two: | Add the numbers in Positions One, Three, Five, Seven, Nine, and Eleven: (6 + 4 + 4 + 2 + 0 + 2 = 18). |
| Step Three: | Multiply the result of Step Two by three: (18 x 3 = 54). |
| Step Four: | Add the numbers in Positions Two, Four, Six, Eight, and Ten: (1 + 1 + 1 + 1 + 2 = 6). |
| Step Five: | Add the results of Step Three and Step Four: (54 + 6 = 60). |
| Step Six: | The Check Digit is the smallest number needed to round the result of Step Five up to a multiple of 10. In this example, the Check Digit is 0. |
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