What Is a Check Digit?
Definition
A check digit is the rightmost digit of a barcode number, used to verify the number was entered correctly. It is derived from the preceding digits by a mathematical rule.
Its purpose is to catch whether the number was corrupted while typing or scanning. When a digit is entered wrong, the computed check digit will most likely differ from the existing one and the error is caught immediately.
Types like EAN-13, EAN-8, UPC-A, GTIN-14 and ITF-14 all include a check digit.
How it's calculated
In the GS1 modulo-10 method, excluding the check digit, digits are taken right to left and multiplied by alternating weights 3 and 1 (the rightmost body digit gets 3). The sum of these products is computed.
Then the check digit = (10 − (sum mod 10)) mod 10. For example, if the sum is 58, then 58 mod 10 = 8, 10 − 8 = 2, so the check digit is 2.
To see these steps with your own digits, use the check digit calculator.
Why weights 3 and 1
The alternating use of weights 3 and 1 was chosen to catch the most common errors: mistyping a single digit and most adjacent digit transpositions are detected by this method.
The weight alignment depends on starting from the right, not on the digit count; that's why the same logic works consistently across all GTINs of 8, 12, 13 and 14 digits.
This simple yet effective method has been used reliably across billions of barcodes for decades.
What it catches and what it doesn't
The check digit catches single-digit errors almost every time. Most adjacent transpositions (e.g. typing 21 instead of 12) are detected too, but some can slip past because they yield the same sum and thus the same check digit.
Complex errors where two or more digits are entered wrong at once also aren't always caught. So a check digit is not a security measure but a check for data-entry integrity.
These limits are known properties of modulo-10 and provide sufficient protection in practice.
Its important limit
A correct check digit only shows the number is internally consistent; it does not prove the number is registered to a real product, owned by you, or a "valid product".
Any tool can produce a number with a valid check digit; that doesn't mean the number was allocated by GS1. To check whether a number is registered, use GS1's official services.
To validate a number by its type, see the GTIN validator or EAN-13 checker.