> For the complete documentation index, see [llms.txt](https://feedshine.gitbook.io/feedshine-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://feedshine.gitbook.io/feedshine-docs/gtin-check-digits-a-practical-guide-for-product-feeds.md).

# GTIN check digits: a practical guide for product feeds

How the GS1 check digit works, a worked EAN-13 example, and the barcode mistakes that get products rejected by Google Shopping.

Google Shopping, Meta and most marketplaces validate the barcode (GTIN) on every product. If the last digit doesn't match, the product gets a warning or is disapproved. This page explains how that last digit works, how to check it, and the mistakes that cause most rejections.

## What a GTIN is

A GTIN (Global Trade Item Number) is the number under a product barcode. There are four lengths:

| Length | Name             | Where you see it                       |
| ------ | ---------------- | -------------------------------------- |
| 8      | GTIN-8 (EAN-8)   | Small packs                            |
| 12     | GTIN-12 (UPC-A)  | North America                          |
| 13     | GTIN-13 (EAN-13) | Most of the world, including Australia |
| 14     | GTIN-14          | Cartons and cases                      |

The last digit of every GTIN is a **check digit**, calculated from the other digits.

## How the check digit is calculated

GS1 uses a mod-10 calculation:

1. Take every digit except the last.
2. Starting from the right, multiply them by 3, 1, 3, 1 and so on.
3. Add the results.
4. The check digit is the number that takes the total up to the next multiple of 10. If the total is already a multiple of 10, the check digit is 0.

### Worked example

EAN-13 body: `400638133393`

From the right: 3×3 + 9×1 + 3×3 + 3×1 + 3×3 + 1×1 + 8×3 + 3×1 + 6×3 + 0×1 + 0×3 + 4×1 = **89**

Next multiple of 10 is 90, so the check digit is **1**. Full GTIN: `4006381333931`.

## The mistakes that get products rejected

| Problem                                | Example                          | Fix                                                                                                                            |
| -------------------------------------- | -------------------------------- | ------------------------------------------------------------------------------------------------------------------------------ |
| Wrong check digit                      | `4006381333932`                  | Re-copy from the packaging or supplier sheet                                                                                   |
| Leading zeros dropped by a spreadsheet | `36000291452` (11 digits)        | Format the column as text; a UPC-A should be 12 digits                                                                         |
| Scientific notation                    | `4.00638E+12`                    | Format as text before pasting                                                                                                  |
| Spaces or dashes                       | `4006381 333931`                 | Strip everything that isn't a digit                                                                                            |
| Restricted prefix                      | `2001234567893`                  | Prefixes 020–029, 040–049 and 200–299 are in-store codes, not global GTINs. Leave the GTIN blank and use a brand + MPN instead |
| Made-up barcodes                       | Any number not licensed from GS1 | Buy a GS1 licence or mark the product as having no GTIN                                                                        |

## Free tools to check them

* [UPC and EAN check digit calculator](https://feedshine.neocities.org/) (single code or a whole list, runs in your browser)
* [Barcode Feed Checker on itch.io](https://feedshine.itch.io/barcode-feed-checker-gtin-upc-ean)
* [Source code on GitLab](https://gitlab.com/feedshine/gtin-check) (MIT, no dependencies)

## Checking every product automatically

[FeedShine](https://feedshine.app/) is a Shopify app that builds Google Shopping, Meta and TikTok product feeds. It runs this check on every variant before the feed goes out and flags the ones that would be rejected.

*Written by the FeedShine team with AI assistance.*


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