GS1 Barcode Basics
GTINs, Application Identifiers, FNC1 and what a GS1 membership actually buys you — the parts you need before printing a logistics label.
On this page
What GS1 is
GS1 is the standards organisation behind the barcodes on retail products and shipping cartons. It defines the numbering systems, the symbol specifications and the data structures, and it licenses company prefixes to members. Almost everything people call "a barcode" in commerce is a GS1 standard.
The identification keys you will meet
| Key | Identifies | Typically printed as |
|---|---|---|
| GTIN-12 / 13 | A trade item (a product) | UPC-A / EAN-13 |
| GTIN-14 | A case or pallet quantity of an item | ITF-14 or GS1-128 |
| SSCC | A specific logistics unit (this pallet) | GS1-128 |
| GLN | A location or legal entity | GS1-128 |
Application Identifiers
A GS1-128 symbol does not just carry a number — it carries labelled fields. Each field starts with an Application Identifier, a two-to-four digit code that says what follows and how long it is. (01) introduces a GTIN, (17) an expiry date, (10) a batch or lot number, (21) a serial number, (00) an SSCC.
The parentheses are for human eyes only. They are printed under the bars and never encoded, which is one of the most persistently misunderstood points in the standard.
FNC1 and why length matters
A GS1-128 symbol begins with a special FNC1 character that marks it as GS1-structured. Some element strings have a predefined length and can be read without a separator; the rest are variable and must be terminated by another FNC1 unless they are the last field in the symbol. Getting this wrong produces a symbol that scans but decodes into garbage at the receiving end. Our GS1-128 generator applies the rule and reports how many separators it inserted.
Check digits
GTINs, SSCCs, GLNs and GSINs all end in a modulo-10 check digit calculated over the preceding digits with alternating weights of 3 and 1. Every one of them is validated before a symbol is drawn, and the validator shows the calculation.
What membership buys
A GS1 company prefix, from which you allocate your own GTINs, and registration in GS1's global registry — which is what large retailers check. Numbers bought cheaply from resellers are typically reassigned from an old prefix that still resolves to someone else's company, and that is exactly what fails verification. Generating a symbol on this site allocates nothing; it only draws a number you are entitled to use.
Which carrier for which job
GS1 element strings can travel in more than one symbol. GS1-128 is the linear carrier used on pallets, cases and logistics labels, read by every warehouse scanner. GS1 DataMatrix carries the same data in a fraction of the space and is the carrier specified for most healthcare marking, including EU FMD and US UDI. Both use identical Application Identifier syntax, so a value that validates in one validates in the other.
What neither does is allocate a number. The symbol is the printed form of an identifier GS1 issued to you; the encoder cannot check entitlement, and nor can any other generator.
Sources
Everything above is defined in the GS1 General Specifications, published at gs1.org, which is the authoritative reference for AIs, symbol placement, dimensions and check-digit calculation.
Written and maintained by the QRBarcodeTools Editorial Team · Last reviewed 2026-08-02 · How we write and check these guides