The last digit of a barcode number is a computer check digit which makes sure the barcode is correctly composed. The check digit is calculated from all the other numbers in the barcode and helps to confirm the integrity of your barcode number. The last digit of all fixed-length, numeric GS1 Identification Keys is a check digit that ensures the integrity of the key.
Use the GS1 check digit calculator to calculate a check digit.
If you don't wish to use a calculator you can manually calculate a check digit.
More about check digits.