By default WooCommerce adds in a SKU custom field to products, but that may not cover your needs, depending on the products you offer. If you’re selling books or magazines ISBN numbers are going to be necessary. Various GTIN (Global Trade Identification Number) options are also important, both for selling and for tracking your products.
How to add GTIN number fields to WooCommerce Products
Or: I need to be able to add GTIN numbers onto the products listed on my store, but WooCommerce only adds an option for SKU?
Fortunately, there is code that can be added to your theme’s function.php that will allow you to use different GTIN options, be they UPC, EAN or ISBN numbers, for the products you’re selling. This is also handy if you happen to be listing your products on Amazon, via a feed.
It is recommended that you try this in a staging environment, which allows you the ability to make sure you have the functionality you’re looking for, without impacting your live site until you’re ready.
Alternative Option: Use a plugin to add GTINs
The option above is great if you’re familiar with customizing your site with code. You can easily drop that code into your themes functions.php file or even as a mu-plugin and get rolling. For those less familiar with using code though an alternative exists. There is a plugin on WordPress.org that can add the field in for you called WooCommerce Add GTIN: UPC, EAN, and ISBN. Just install this plugin like any other one and you’ll be good to go.