If you sell on Amazon FBA, two barcode types will determine whether your inventory gets received correctly or sits stranded at a fulfillment center: the FNSKU and the UPC. They look nearly identical on a label - both are Code 128 barcodes with a number underneath - but they represent fundamentally different concepts, and Amazon's 2026 policy overhaul has changed when each one is required. This guide covers what you actually need to know.
FNSKU vs UPC at a glance
| FNSKU | UPC / GTIN | |
|---|---|---|
| Issued by | Amazon (free, automatic) | GS1 (paid registration) |
| Identifies | Product + specific seller account | Product globally, regardless of seller |
| Format | X00xxxxxxx (10 chars, Code 128) | 12 digits (UPC-A) or 13 digits (EAN-13) |
| Unique per seller? | Yes | No - shared by all sellers of same product |
| Required after March 2026 | Resellers: yes (mandatory) | Brand owners with Brand Rep role: yes (permitted) |
| Commingling risk | None (segregated inventory) | Eliminated post-March 2026 |
What is an FNSKU?
FNSKU stands for Fulfillment Network Stock Keeping Unit. It is Amazon's proprietary identifier - a barcode that encodes both a product and the specific seller account that owns it. Format: always starts with X00, 10 characters total (for example, X001KR74B2). It is encoded as a Code 128 barcode.
You cannot purchase an FNSKU or generate one externally. It is created automatically when you set up FBA inventory in Seller Central and tied to your account. Two sellers listing the same product have different FNSKUs for that product - the code identifies the seller, not just the item. When an Amazon receiving associate scans an FNSKU, the system knows whose unit it is, which listing it belongs to, and which seller earns the sale. This is the mechanism that makes per-seller inventory attribution possible in a warehouse handling inventory from tens of thousands of sellers simultaneously.
What is a UPC?
A UPC (Universal Product Code) is a 12-digit UPC-A barcode identifier issued through the GS1 standards system - specifically GS1 US for North American brands. The 13-digit version used outside North America is called an EAN-13. Both are GTINs (Global Trade Item Numbers). A GTIN identifies a product globally - it is brand-owned, universally recognized, and not specific to any retailer.
The FNSKU is account-specific; the UPC is not. If twenty different sellers all list the same product, they share one ASIN (Amazon's catalog identifier) and one UPC, but each has a unique FNSKU. That single distinction between "identifies the product" and "identifies the product owned by this seller" is the root of every FNSKU vs UPC question.
To obtain a legitimate UPC, purchase directly from GS1 US - not from third-party resellers selling cheap codes in bulk. Amazon cross-checks GTINs against the GS1 database and verifies that the registered brand name matches your seller account. Barcodes registered to someone else's company will trigger listing suppression. GS1 US offers two options: a single GTIN for $30 (one-time, no renewal) for brands with one to three products, or a GS1 Company Prefix starting at $250 with annual renewal, which gives you a unique prefix to self-assign product numbers across your catalog.
What was commingling and why did it end?
For years, Amazon offered sellers a choice: label every unit with an FNSKU (segregated inventory), or rely on the manufacturer's UPC and let Amazon pool your units with physically identical units from every other seller (commingled or stickerless inventory).
Commingling had one appeal: no label application required before shipping. The downsides were significant. Your buyer could receive a unit from a completely different seller's inventory - and if that seller's batch was counterfeit, damaged, or sourced from a different region, it shipped under your listing and your reviews took the hit. Commingling was effectively a gift to bad actors, and private label sellers who actually owned their products resented having their inventory mixed with cheap knock-offs they had no control over.
Amazon announced the formal end of commingled/stickerless inventory effective March 31, 2026. All units arriving at fulfillment centers after that date must be uniquely attributable to a specific seller. The commingling argument is over.
The 2026 rules: two tracks
The requirement after March 31, 2026 splits based on your role in Amazon Brand Registry:
Track A - Resellers (no Brand Representative role): FNSKU labeling is mandatory on every unit. This includes retail arbitrage sellers, wholesale resellers, and any seller without the Brand Representative role in Brand Registry. Even if the manufacturer's UPC is printed on the product, you must apply an FNSKU sticker before shipping to Amazon.
Track B - Brand owners with Brand Representative status in Brand Registry: You can ship using only the manufacturer's printed barcode. Amazon uses virtual tracking - not physical label pooling - to attribute inventory to your account. This is not a return to commingling; it is per-brand attribution without the label application step. The catch: this only applies if each UPC maps to a single ASIN. One UPC, multiple ASINs - you are back to FNSKUs.
One clarification worth making explicit: not every Brand Registry enrollee qualifies for Track B. Only the Brand Representative role grants the barcode flexibility. A seller enrolled with a Reseller role in Brand Registry is still on Track A and must apply FNSKUs.
Also: Amazon discontinued its FBA prep and labeling service in the US effective January 1, 2026. Amazon will no longer apply your labels at the fulfillment center. All labeling must be completed before the shipment is created. Third-party prep centers charge roughly $0.35-$1.20 per unit for the same service if you cannot label at source.
How to get your FNSKU
FNSKUs are free and generated automatically through Seller Central:
- Log into Seller Central. Go to Settings > Fulfillment by Amazon > FBA Product Barcode Preference. Set the default to "Amazon Barcode" (FNSKU).
- Go to Inventory > Manage FBA Inventory. Add the FNSKU column to your view. The X00 code appears once the listing is active in FBA.
- To print labels: select the product, choose "Print item labels" from the action dropdown. Pick your label format (30-up Avery-compatible sheet for desktop printers, or thermal roll format), specify quantity, download PDF, and print.
Each product variation - size, color, configuration - has its own FNSKU because each is a distinct ASIN and a distinct listing in Amazon's catalog. Generate and print labels separately for each variant. Applying a Large FNSKU to a Medium unit is one of the most common and most expensive FBA receiving errors there is.
Label placement and print quality
Amazon's label requirements are not suggestions:
- Minimum size: 1 inch x 2 inches. Amazon's default PDF format generates 30-up Avery-compatible labels at roughly 1" x 2-5/8".
- Placement: flat surface on the outermost layer of the sellable unit. Never on curves, corners, seams, or textured surfaces - bar spacing distorts on any curved surface and the label will fail scanning. A polybagged unit gets its label on the outside of the polybag, flat face only.
- Cover other barcodes: if using FNSKU, cover every other scannable barcode on the unit. A visible UPC alongside an FNSKU means Amazon may scan the wrong code.
- Print quality: black ink on white, non-reflective background, minimum 300 DPI. Thermal direct printers (Zebra, Dymo, Rollo) are the industry standard - no ink management, consistent output, designed for exactly this use case. Test-scan a label from every new batch before committing to a full print run.
- Label contents: the Code 128 barcode, the FNSKU in human-readable text below it, product description, and condition ("New").
The mistakes that cost sellers real money
Labeling only the outer case, not individual units. Amazon scans individual units at receiving. Every unit inside a case pack needs its own label. Miss one and those units are unsellable until corrected.
Wrong FNSKU on a size variant. Applying a Large FNSKU to a Medium unit creates an inventory attribution error that is painful and slow to resolve. For multi-variant products, generate labels per variant, label the print file clearly, and verify before batch printing.
Barcode on a curved surface. Any flex or curve distorts bar spacing and the label fails at receiving. Cylindrical containers get their label on a flat side panel, full stop.
Buying resold GTINs. Amazon verifies GTINs against the GS1 database. A mismatch between the registered brand name and your seller account triggers listing errors and potential removal. Buy from GS1 US directly.
Sending unlabeled inventory after January 2026. Amazon's FBA prep and labeling service is gone. Unlabeled units arriving at a fulfillment center are rejected with no on-site correction option.
The bottom line
If you are a private label brand owner with Brand Representative status in Brand Registry, the 2026 change works in your favor: print your GS1-registered UPC on your packaging at the factory and skip the label application step. Your per-unit cost drops and your supply chain simplifies. If you are a reseller - retail arbitrage, wholesale, authorized distributor - the FNSKU is now mandatory on every unit. Apply at the supplier, at a prep center, or yourself before shipping. Build the cost in, not around it.