The all-important check digit ensures our numbers are correct
All Global Trade Item Numbers (GTINs) and Global Location Numbers (GLNs) end with a computer-generated check digit. This digit is the result of an algorithm (called the Modulo-10 algorithm) which uses the other numbers in the GTIN or GLN to ensure its correct composition.
You can use the below check digit calculator to obtain your barcode check digit.
If you want to understand how the check digit is calculated then take a look at the attached PDF, which will give you the step by step break down.