SKU vs UPC vs ASIN vs FNSKU: What Each Number Is For

The four numbers side by side

The short version: the UPC is the product's public identity. The SKU is your private one, and the ASIN and FNSKU are Amazon's internal ones.

SKU: the code you control

A SKU, or stock keeping unit, is a code you create to identify products in your own inventory. Amazon's seller guide stresses the difference: a SKU is tied to a specific business, while a UPC stays attached to the product no matter who sells it. If you and a competitor sell the same item, you'll have different SKUs but the same UPC (Sell on Amazon SKU guide).

No standards body issues SKUs, so the format is up to you. Readable codes work best, such as CND-CED-08 for a cedar candle in the 8 oz size. Give each sellable variation its own SKU, and leave out details that can change, like price or supplier.

On Amazon, the SKU is how your offer links to a product page. If you don't supply one, Amazon creates and assigns a SKU for you. To scan SKUs in your own stockroom, encode them as Code 128 barcodes with the barcode generator. Setting up the rest of that system is covered in barcode inventory for small businesses.

UPC: the product's global identity

A UPC barcode carries a GTIN-12, a Global Trade Item Number. GS1 US describes the GTIN as the number that identifies the product and the UPC as the barcode that lets a scanner read it (GS1 US FAQ). Outside North America the same job is usually done by a 13-digit GTIN in an EAN-13 barcode (What is a GTIN?).

Unlike the other three numbers, a UPC isn't made up by you or by Amazon. The brand owner builds it from a number or prefix licensed through GS1, and GS1 US says every variation, such as size, color, style or package count, needs its own GTIN.

Amazon's guide to product IDs sets out who is responsible. If you resell a manufacturer's product, ask the manufacturer for its GTIN. If you create and own the product, get the GTIN through GS1 (Sell on Amazon GTIN guide). The same guide says some products without a GTIN may qualify for a GTIN exemption, which requires a product title and images of every side of the product and its packaging. [VERIFY] The licensing route is explained in how to get UPC barcodes. This comparison is general information, not legal advice on licensing.

ASIN: Amazon's catalog number

An ASIN, or Amazon Standard Identification Number, is a 10-character code of letters and numbers assigned to each product in the Amazon store. Most start with "B0," while book ASINs usually start with a number (Sell on Amazon ASIN guide).

Amazon assigns a new ASIN when someone creates a new product detail page. If the product already has a page, you add your offer to the existing ASIN rather than creating a new one. That is why every seller of the same product shares one ASIN.

The UPC and the ASIN connect at listing time. You enter the product's GTIN, Amazon checks whether a matching page exists, and you either join that page or create a new one. The ASIN means nothing outside Amazon, so a store checkout or another marketplace has no use for it.

FNSKU: Amazon's label for your units

An FNSKU (Fulfillment Network Stock Keeping Unit) only matters if you use Fulfillment by Amazon. Amazon's FBA guide describes three types of barcodes. The two that matter for most sellers are:

• Manufacturer barcodes, such as a UPC, EAN, JAN or ISBN. These are the default, and Amazon uses them to track your inventory unless you change your barcode setting. When several sellers send in units with the same manufacturer barcode, Amazon may fill an order from whichever units are closest to the customer.
• Amazon barcodes, which print the FNSKU. Amazon requires them for products not tracked by manufacturer barcode, used items, products without a scannable manufacturer barcode, media, restricted or dangerous goods, and some consumable and topical products. [VERIFY]

The same guide says an FNSKU label should show a scannable barcode, the FNSKU, the product title and the condition, and that any original barcodes should be covered so they can't be scanned instead (Sell on Amazon FBA prep guide).

Because an FNSKU is tied to your seller account, it keeps your units separate from other sellers' units of the same ASIN, and sellers commonly report that FNSKUs begin with "X." [VERIFY] Amazon's FBA barcode rules have also been changing, and industry reports describe a 2026 move to require FNSKU labels from sellers not enrolled in Brand Registry. Check your barcode setting in Seller Central before you send a new product to FBA. [VERIFY]

One product, four numbers: an example

Take the 8 oz cedar candle again, sold in your own shop, on Amazon through FBA, and at a local boutique:

The UPC in this example uses 614141, a prefix GS1 uses in its documentation examples, so don't print it on real products. The boutique scans the UPC at its register and never sees the ASIN or FNSKU. Your stockroom scans the SKU, and Amazon's warehouse scans whichever barcode your FBA setting says to use. If you also ship cases, those get their own case-level numbers, which is a separate topic.

Bottom line

Use the SKU for your own inventory, and keep it unique within your business. Use the UPC, licensed to your brand, as the one identity that follows the product into every store and marketplace. Let Amazon handle the ASIN and FNSKU. They only exist inside Amazon, so never print an ASIN or FNSKU on packaging meant for other retailers.