FNSKU vs ASIN vs UPC vs SKU.

The four identifiers side by side

The “in Amazon’s words” column quotes the field descriptions in Amazon’s Selling Partner API models, which is the closest thing to a formal definition Amazon publishes openly. Everything about shape beyond those quotes is described in the sections below with its source.

SKU: yours, and it cannot be recycled

An Amazon staff member explaining the relationship in the Seller Forums says a SKU is the identifier for a specific listing or offer in your inventory, tied to your seller account and internal to it. You can make it anything you want or use characters Amazon generates. One rule matters: as long as a merchant SKU exists it cannot be reused for a different ASIN, even if the earlier offer was deleted, so a new product needs a new SKU. Once an FBA listing is created you cannot change its merchant SKU.

That is why SKU naming deserves care before you upload hundreds of rows. The SKU naming convention guide and variant and SKU collision checker help with that.

ASIN: Amazon’s product code

Amazon describes an ASIN as a unique 10-character alphanumeric code assigned to every product in its store, typically starting with B0 or, for books, a number (examples B07FZ8S74R and 0451524934). It identifies the product, whereas the FNSKU is tied to your own inventory. Our ASIN validator covers the format and the ISBN-10 link in depth, so this page does not repeat it.

FNSKU: your units inside Amazon’s network

Amazon’s SP-API describes the FNSKU field only as “Amazon’s fulfillment network SKU identifier”. The Amazon staff post adds the useful part: an FNSKU is similar to a manufacturer barcode but tied to your inventory in the fulfillment network for FBA offers, linked to your ASIN when you create the FBA listing, and, for Amazon barcodes, “a unique number starting with an X”. It is seller specific and not commingled with other sellers’ inventory, so no two sellers share an Amazon-barcode FNSKU.

The ten-character length is the figure commonly reported in seller guides; Amazon documents no length, we have not verified it against an Amazon page, and the post does not say what follows the X. The tool therefore calls a ten-character X-code “the shape described for an FNSKU” and never claims more.

There is a trap in the same post. If you use the manufacturer’s barcode (UPC, EAN, JAN or ISBN) instead of an Amazon barcode, your Manage Inventory page shows the ASIN of the product in the FNSKU position. So a ten-character code that is not X-prefixed could be an ASIN or an FNSKU, and only the place you copied it from tells you which. The barcode preference has to be set to Amazon Barcode before the FBA listing is created, or it defaults to Manufacturer Barcode.

UPC: the manufacturer’s number

GS1 says EAN/UPC barcodes are printed on virtually every consumer product in the world. A UPC-A carries a 12-digit GTIN whose last digit is a check digit computed with GS1’s mod-10 method (weights 3 and 1 from the right), which the tool applies to 8-, 12-, 13- and 14-digit inputs. Our EAN-13 and UPC-A check digit validator goes deeper, and the GTIN exemption guide covers products that have no UPC.

Worked example

Take B07FZ8S74R, the example in Amazon’s own ASIN article. Ten characters, begins B0: the tool calls it an ASIN. 0451524934 is ten digits and passes the ISBN-10 weighted check, so it reads as a book-style ASIN. 036000291452 is 12 digits with a correct GS1 check digit, so it reads as a UPC-A. X001ABC123 is ten characters beginning with X, so it fits the shape described for an FNSKU, although it is a made-up value. BLUE-TSHIRT-M is none of those, so it reads as a seller SKU, which could be anything you chose.

The common failure is typing one digit wrong in a UPC. The tool then says the length fits a GTIN but the check digit should be a specific digit, which is the clue that you have a typo and not a different kind of code.

Which one goes where

Your SKU belongs in listing and inventory files and on your own bins and pick lists. The ASIN belongs wherever you need to point at the Amazon product page. The FNSKU belongs on the unit label when Amazon requires an FNSKU label; the older inbound API even has a barcode instruction value, RequiresFNSKULabel, defined as “the FNSKU label should be applied to the item”. The UPC belongs on the product packaging and in the listing’s product-ID field. Mixing them up usually shows as a listing that will not match or a unit that is received as the wrong item.

What this page does not cover

• It does not look an identifier up on Amazon, so it cannot say whether an ASIN or FNSKU exists.
• It does not state a length rule for FNSKUs or SKUs that Amazon has not published.
• It does not advise on FBA labelling policy; Amazon’s current barcode requirements in Seller Central govern, and they change.
• It does not store, send or log what you type.