The last digit of a GTIN is a check digit that is calculated based on the preceding numbers. Having a wrong check digit renders the GTIN invalid.
Whether you have a GTIN 13 or GTIN 14, the last digit is always a check digit. GS1 offers the online Check digit calculator with which you can verify what the last digit should be.
Check digit calculator
Go to the GS1 Check digit calculator and enter the first 12 characters of a GTIN 13 or the first 13 characters of a GTIN 14 in order to find out what the correct last digit should be. When your GTIN has a different check digit, it is invalid and you'll have to replace it with a valid one. You could, for example, simply replace the check digit with the valid one you just discovered.
