What is a GTIN (Global Trade Item Number)?

If you sell other brands' products, the GTIN already exists: it is the number under the barcode on the box. It is not your internal reference, the SKU, nor the manufacturer's catalog reference, the MPN, nor exactly the EAN code, which is the barcode symbol that carries it. It matters to you because it is how Google and the other channels know your product is the same one another store sells: without it, or with a wrong one, Google warns that your product may have limited visibility.⁠^{[1]Google Merchant Center Help. GTIN [gtin]}

How it works

GS1, the organization that maintains the standard, defines GTINs as unique, universal and unambiguous numberings that identify trade items, meaning products or services that have a price and can be invoiced and bought at any point in the supply chain. A GTIN can only be generated from a GS1 company prefix, and the last digit is always a check digit.⁠^{[2]GS1 Spain (AECOC), in Spanish. Qué es un código GTIN} GS1 US sums it up this way: it is a number GS1 issues exclusively and that all retailers, distributors and marketplaces in the world use to identify the same product, while each store gives it its own SKU for its inventory.⁠^{[3]GS1 US. What is a GTIN?}

There are four lengths, each with its use:⁠^{[2]GS1 Spain (AECOC), in Spanish. Qué es un código GTIN}

The check digit is what makes it possible to detect a mistyped number. GS1 Spain explains the calculation in three steps: number the digits from right to left, multiply by 3 the ones in odd positions and by 1 the ones in even positions, add up the results and subtract that sum from the next multiple of ten. What is left is the check digit.⁠^{[4]GS1 Spain (AECOC), in Spanish. Cálculo del dígito de control}

At Google the GTIN comes in through two doors. In the product feed for Google Merchant Center, Google recommends it for every product that has a manufacturer-assigned GTIN, ideally together with the MPN, and sets clear conditions: only if you are sure it is correct, with the check digit properly calculated, outside the restricted ranges (prefixes 02, 04 or 2) and the coupon ones (05, 98 or 99), and a different one for each color or size variant.⁠^{[1]Google Merchant Center Help. GTIN [gtin]} In the structured data of the product page it goes in the Product node. Schema.org defines the gtin property for values of 8, 12, 13 or 14 digits with a valid check digit, and presents it as the generalization of gtin8, gtin12, gtin13 and gtin14.⁠^{[5]Schema.org. gtin} Google accepts the generic one, but recommends the most specific one that fits the product, because it is the most exact representation, and it only accepts the number, not the URL form.⁠^{[6]Google Search Central. Merchant listing (Product, Offer) structured data}

In a store

What you need to know is where each product's number comes from. Fictitious example: the store and the products are invented, and the last column applies Google Merchant Center's rules.⁠^{[1]Google Merchant Center Help. GTIN [gtin]}

And the check digit, calculated by hand with GS1's method on a made-up number: 2001234567893.⁠^{[4]GS1 Spain (AECOC), in Spanish. Cálculo del dígito de control} It starts with 2 on purpose, a prefix from the restricted range that Google does not accept in a feed,⁠^{[1]Google Merchant Center Help. GTIN [gtin]} so nobody mistakes it for a real product.

Digits (without the check digit):   2  0  0  1  2  3  4  5  6  7  8  9
Weight, from the right:             1  3  1  3  1  3  1  3  1  3  1  3
Product:                            2  0  0  3  2  9  4 15  6 21  8 27
Sum: 97 · next multiple of ten: 100 · check digit: 100 - 97 = 3

If your catalog has a GTIN that is missing a digit or whose last digit does not match this calculation, the number is wrong, and the right thing is to ask the manufacturer for it, not to fix it by eye.

How to check it

1. Count the digits. A valid GTIN has 8, 12, 13 or 14, with no letters.⁠^{[5]Schema.org. gtin}
2. Recalculate the check digit with GS1's calculator or with the three-step method.⁠^{[4]GS1 Spain (AECOC), in Spanish. Cálculo del dígito de control}
3. Look it up in Verified by GS1. With the product's number, the lookup returns the data of the company that owns the brand.⁠^{[7]GS1 Spain (AECOC), in Spanish. Códigos de barras: cómo consultar y verificar los códigos de barras de los productos} If it is not the brand you sell, something is wrong.
4. Look at the prefix. Those starting with 02, 04 or 2 are restricted circulation, and those starting with 05, 98 or 99 are coupons: Google does not accept them as a GTIN.⁠^{[1]Google Merchant Center Help. GTIN [gtin]}
5. Compare variants and channels. Each size and each color, its own number; and the same number in the feed and in the page's gtin13 markup.⁠^{[1]Google Merchant Center Help. GTIN [gtin]}⁠^{[6]Google Search Central. Merchant listing (Product, Offer) structured data}

Related concept: EAN code

Many people say "the EAN" when they mean the GTIN, and for day-to-day purposes it makes no difference. The difference is this: the GTIN-13 is the number; the EAN-13 is the barcode that represents it, and GS1 Spain warns that one is sometimes requested in place of the other.⁠^{[2]GS1 Spain (AECOC), in Spanish. Qué es un código GTIN} Google Merchant Center treats them as the same for the feed: an EAN is, to it, a 13-digit GTIN-13.⁠^{[1]Google Merchant Center Help. GTIN [gtin]} Where the difference matters is on the physical label: the symbol can be printed badly or at a size the scanner cannot read, and the number still be correct. The EAN code entry covers the symbol; this one, the number.

To go on, the structured data reading path puts the rest of the terms in order, and the Product entry explains where the GTIN and the other identifiers go on the page.