SKU vs UPC vs GTIN vs ASIN vs FNSKU: What Each Code Is For
Online sellers juggle a lot of codes. Each one answers a different question: what is this product, who is selling this unit, and where is it in my stockroom?
At a glance
| Code | Who creates it | What it identifies | Do you need it? |
|---|---|---|---|
| SKU | You | A product variant in your inventory | Yes, always |
| GTIN (UPC, EAN) | The brand owner, using numbers licensed from GS1 | A trade item, the same everywhere it's sold | If the product has one, or if you're the brand and a marketplace or retailer requires it |
| ISBN | The publisher, from the national ISBN agency (Bowker in the US) | A book edition and format | For books that have one |
| ASIN | Amazon | A product page in Amazon's catalog | Only on Amazon, and Amazon assigns it |
| FNSKU | Amazon | Your specific units stored in Amazon's warehouses | When you sell through Fulfillment by Amazon, in most cases |
SKU: your code
A SKU is the only code on this list you invent. It identifies a variant in your own stock: size, color, condition, bundle. Two sellers can sell the same UPC with completely different SKUs, and that's fine. It's also the code that ties your listings together across marketplaces. Build one with our SKU numbering guide.
GTIN, UPC and EAN: the product's global ID
GTIN (Global Trade Item Number) is the umbrella name for the numbers under retail barcodes. The 12-digit UPC common in North America is a GTIN-12. The 13-digit EAN used in much of the world is a GTIN-13. GTINs are managed by GS1, a nonprofit standards organization, and the brand owner assigns them from a company prefix licensed from GS1.
If you resell a branded product, use the GTIN already printed on it. If you're the brand, see how UPCs and GTINs work before buying any.
ASIN: Amazon's catalog number
Amazon gives every product page in its catalog a 10-character ASIN (books use their ISBN-10). When you list an item that's already in the catalog, you join the existing ASIN. You never create an ASIN directly; Amazon assigns one when a new product page is created.
FNSKU: Amazon's warehouse label
The FNSKU (fulfillment network SKU) identifies your units inside Amazon's fulfillment centers, so Amazon knows which seller's inventory it's shipping. For sellers using Fulfillment by Amazon, the rules on when you must label units with an FNSKU versus using the manufacturer barcode changed in 2026. Read Amazon FBA labels and FNSKUs for the current details and links to Amazon's own help page.
How they connect in practice
Imagine a reseller with three identical, new-in-box coffee mugs:
- The UPC on the box identifies the mug model, the same for every store.
- Her SKU,
MUG-XMS-RED-0007, identifies those mugs in her stock, at binB-1-3. - On Amazon, the mugs are listed on an existing ASIN, and units she sends to FBA carry an FNSKU label.
- On eBay, the listing includes the UPC as the product identifier and her SKU in the custom label field.
The one that matters most is the one you control. Marketplaces come and go. Your SKU stays the same in every channel and every spreadsheet.
Common questions
Do I need a UPC to sell on eBay?
Only for items that have one. eBay asks for product identifiers like UPC, EAN, ISBN or the brand's part number when they exist. Handmade, vintage and unbranded items usually don't have them.
Is an ASIN the same as a UPC?
No. Amazon assigns an ASIN to each product page in its catalog. A UPC (a type of GTIN) identifies the product itself everywhere it's sold.
Can I make up my own UPC?
No. A real UPC comes from a GS1 company prefix or a single GTIN bought from GS1. Made-up numbers can collide with other companies' products.
Sources
- GS1 US: About GTINs, UPCs and barcodes
- Amazon Seller Central: FBA product barcode requirements
- eBay developer docs: Item specifics and product identifiers
Marketplace and GS1 rules change. Check the linked source before relying on a specific requirement or price.