What Is GTIN?
Definition
GTIN (Global Trade Item Number) is the GS1 family of numbers that uniquely identify trade items globally. EAN, UPC and ITF-14 numbers are all GTINs; GTIN is their shared umbrella concept.
A GTIN is not a barcode type but an identifier. The same GTIN can be printed with different symbologies (like EAN-13 or ITF-14); the barcode is the visual form that carries the number.
To validate a GTIN and learn its type, use the GTIN validator.
GTIN lengths
There are four GTIN lengths: GTIN-8 (EAN-8, small products), GTIN-12 (UPC-A, North American retail), GTIN-13 (EAN-13, most common) and GTIN-14 (case/carton level).
Which length to use depends on the product type and packaging level. An individual retail unit usually carries GTIN-13 (or GTIN-12), while the case that groups those units carries GTIN-14.
Each length belongs conceptually to the same GTIN family and its check digit is computed with the same method.
GTIN-14 and normalization
GTINs of different lengths can be "normalized" to 14 digits by prefixing zeros. For example a GTIN-13 is stored as 14 digits by adding a 0; this GTIN-14 format is the standard way to store items uniformly in databases.
This lets an ERP or inventory system hold 8, 12, 13 and 14-digit numbers in a single 14-digit column, simplifying matching. The GTIN validator gives you this padded form.
Normalization is only a storage format; when printed as a barcode, the natural length of the appropriate symbology is used.
Check digit and validation
Every GTIN's last digit is a check digit computed with the GS1 modulo-10 method: digits are multiplied right to left by weights 3 and 1, summed, and the last digit is set accordingly. For the step-by-step math, see the check digit calculator.
The check digit catches whether the number was corrupted during typing or scanning. But this is only a technical integrity check.
A correct check digit does not show the number is registered to a real product; it only shows the number is internally consistent.
Relationship to EAN/UPC
EAN-13 is a GTIN-13, UPC-A is a GTIN-12, and EAN-8 is a GTIN-8. So what we casually call a "barcode number" is actually a GTIN; the different names reflect different lengths and regional conventions.
The multitude of names comes from history; GS1 unified all these systems under a single GTIN framework.
For details on individual types, read what is EAN-13 and what is UPC.
Important note
A GTIN having a correct check digit does not mean the number is registered to a real product or owned by you. Any tool can produce a GTIN with a valid check digit.
To obtain a real, unique GTIN you apply to an authority such as GS1; this site does not assign official GTINs. For registration checks, use services like Verified by GS1.
In short: a valid GTIN ≠ a registered product. Keeping this distinction in mind prevents misunderstandings, especially in e-commerce and supply chains.