Quick Answer
An FNSKU (Fulfillment Network Stock Keeping Unit) is Amazon’s barcode for one seller’s units inside its warehouses. It starts with an X (X00 on the label). Since March 31, 2026, Amazon’s FBA barcode FAQ says resellers must put an FNSKU label on every unit they send to FBA, while brand owners in Brand Registry can ship eligible products with the manufacturer barcode instead.
If you sell through FBA, you’ve probably been asked to “print SKU labels” and wondered whether you really need them, or why the barcode already on the box isn’t enough. For years, the answer was often “you don’t.” That changed on March 31, 2026.
This guide explains what an FNSKU is, how it differs from an ASIN, UPC and SKU, what Amazon changed in March, who needs labels now, and the exact label rules: size (in inches and millimeters), printers, and where the sticker goes.
About the Author: Diana Walsh writes for the SellerSonar editorial team about Amazon fulfillment, account health, and the policies behind both. She checks every rule in her articles against Amazon’s own Seller Central help pages before publishing.
What is an FNSKU on Amazon?
FNSKU stands for Fulfillment Network Stock Keeping Unit. An Amazon staff account explained it on the Seller Forums as an identifier that’s “similar to a manufacturers barcode, but is tied to your inventory specifically in the Amazon Fulfilment Network for FBA offers.”
The key word is your. A product’s ASIN is the same for every seller on that product page. An FNSKU belongs to one seller’s inventory of that ASIN. Two sellers offering the same kettle share one ASIN but have two different FNSKUs, and per the same Seller Forums post, “No two sellers will share an Amazon barcode FNSKU’s.”
That’s the whole point. When a unit arrives at a fulfillment center, the FNSKU tells Amazon whose stock it is, so it sells, ships, returns and gets reimbursed against the right account.
What the X00 prefix means
Amazon barcodes are easy to spot. Amazon’s FBA barcode requirements describe them as “FNSKU labels that start with an X,” and the barcode FAQ calls them “X00 labels.” If you see a barcode starting with X00 on a product (including on returned or liquidated items), it’s an Amazon warehouse label, not a manufacturer UPC. It only means something inside Amazon’s network.
FNSKU vs ASIN vs UPC vs SKU
Four codes, one product, four different jobs:
| Code | Who creates it | What it identifies | Used where |
|---|---|---|---|
| ASIN | Amazon | The product page | Amazon’s catalog, shared by every seller |
| FNSKU | Amazon | Your units of that product | Amazon’s fulfillment centers only |
| UPC / EAN | GS1, licensed by the brand | The product itself | Every store and marketplace |
| SKU | You | Your offer | Your own inventory and reports |
If you want the longer version of the UPC side, our guide on what a UPC code is covers the digits, the cost, and Amazon’s GS1 check.
What changed on March 31, 2026
On December 8, 2025, Amazon posted on the Seller Forums that “commingling practices will end effective as of March 31, 2026.” That announcement applied the new rules to inventory shipped on or after that date.
Commingling is what made the manufacturer barcode work as a shortcut. Amazon’s FAQ defines it as fulfilling “customer orders using exact product matches from the closest available inventory, even if that inventory belonged to a different seller.” If you and five other sellers sent in the same UPC-coded product without stickers, Amazon could ship any of those units to any of your customers.
That’s over. Per Amazon, “customers who buy from you will only receive products from your inventory, including returns and removals.” The labeling rules changed to match.
If you’re a brand owner
Brand owners get the easier deal. You can keep shipping with the manufacturer barcode, no FNSKU sticker, if the product meets all of these conditions from Amazon’s manufacturer barcode page:
- It’s in new condition.
- It has “a single, scannable UPC, EAN, or ISBN barcode matching a single ASIN in the Amazon catalog.”
- Its brand is enrolled in Amazon Brand Registry and you hold the brand representative role.
There’s a fourth step people miss. Amazon says “Enrolling your brand in Brand Registry doesn’t automatically connect your products to your brand.” You have to register each product to the brand as well. And if a product has a GTIN exemption instead of a real UPC, it needs FNSKU labels anyway.
To switch, set your FBA product barcode preference to manufacturer barcode. Amazon says it takes about 24 hours to take effect “and it will apply only to new offers.”
Existing offers are converted separately. Amazon’s guide to switching to manufacturer barcodes gives three ways: convert eligible offers in bulk with a download file, convert one offer at a time in Send to Amazon, or create a new SKU. If an offer already has units in a fulfillment center under an Amazon barcode, you either list it again under a new SKU or sell through (or remove) that stock first.
💡 Expert Tip: Check your product registration in Brand Registry before you change the barcode preference. If a product isn’t linked to your brand, the manufacturer barcode option won’t be available for it, and you’ll be printing stickers for that SKU whether you planned to or not.
If you’re a reseller
This is the big one. Amazon’s FAQ is explicit: “all resellers, including authorized resellers, must use Amazon barcodes when sending inventory to our fulfillment centers.” Even when the product already has a perfectly good UPC.
What happens if you don’t? Per the same FAQ, reseller inventory that arrives without Amazon barcodes “will be considered defective.” Amazon says it will try to fix the labeling and flag it on your Inbound Performance Dashboard, but units that aren’t properly labeled “are not eligible for reimbursement.” Lose one in the warehouse and you eat the cost.
Two things soften the transition:
- Stock already at Amazon is fine. Inventory that was in, or on its way to, fulfillment centers before March 31, 2026 needs no action and keeps selling under its original SKU.
- Low-inventory-level fees are waived at the FNSKU level in the US, Canada, Germany, France, Italy and Spain while affected ASINs move over, according to the FAQ.
Changing to Amazon barcodes on an existing offer means creating a new SKU. Amazon says your ratings and reviews won’t be affected, because “those metrics are measured for an ASIN.”
Do you need an FNSKU label? A quick check
Run your product through these questions. If any answer is “no,” you need an FNSKU label on every unit.
- Are you the brand owner, with the brand representative role in Brand Registry?
- Is the product registered to your brand?
- Is it new?
- Does it have a single scannable UPC, EAN or ISBN that matches exactly one ASIN?
- Is that a real GTIN, not a GTIN exemption?
Five yeses, and you can use the manufacturer barcode. Anything else, and it’s an FNSKU label. That includes every reseller, every used item, and every product without a barcode.
FNSKU label requirements
Amazon’s label rules haven’t changed with the March update. All of the specs below come from Amazon’s Use Amazon barcodes for FBA help page.
Label size
Labels must be between 1 x 2 inches and 2 x 3 inches. Amazon gives 1 x 3 inches and 2 x 2 inches as examples that fit.
In millimeters, that range runs from about 25.4 x 50.8 mm to 50.8 x 76.2 mm. If you print on A4 sheets with a laser printer, Seller Central’s label tool supports these layouts:
| Labels per sheet | Label size |
|---|---|
| 21 | 63.5 x 38.1 mm (A4) |
| 24 | 63.5 x 33.9 mm, 63.5 x 38.1 mm, 64.6 x 33.8 mm, 66.0 x 33.9 mm, 70.0 x 36.0 mm, 70.0 x 37.0 mm (A4) |
| 27 | 63.5 x 29.6 mm (A4) |
| 30 | 1 x 2 5/8 inches (US letter) |
| 40 | 52.5 x 29.7 mm (A4) |
| 44 | 48.5 x 25.4 mm (A4) |
When you print, set the printer’s scaling to None or 100%, per Amazon’s instructions. If it shrinks the PDF to fit the page, the barcodes come out the wrong size.
What the label must show
Your printed label has to match the format Seller Central generates:
- The FNSKU (or ASIN), the product name, and the item condition
- White space of 0.25 inch on the sides and 0.125 inch on the top and bottom, both around the barcode and between the content and the label’s edge
The same rules apply if you print the Amazon barcode straight onto your packaging.
Printers and paper
Amazon’s tools are built for thermal printers, and laser printers work too. Never use an inkjet: Amazon’s page says it outright. Its own fulfillment centers use Zebra GX430t printers on a direct thermal setting.
The rest is about making sure the label still scans months later. Amazon wants every label “readable and scannable for 24 months,” printed at 300 dpi or better, in black ink on white, non-reflective labels with removable adhesive. Test a few labels with a scanner now and then, and clean the print head. (Faded thermal labels are the boring problem nobody plans for, honestly.)
Where to put the label
- Cover every other visible barcode on the unit, except serial number barcodes and Transparency labels.
- Don’t cover warning, safety or compliance stickers.
- Keep it off curves and corners, and at least 0.25 inch from the packaging edge.
- Make sure it still scans after any polybag or bubble wrap goes on.
- In a case pack, label every unit and remove any barcodes on the case itself.
Every FNSKU must also map to one product. Amazon’s packaging requirements say each size or color needs its own FNSKU.
How to print FNSKU labels in Seller Central
There are two places to do it:
- While creating a shipment. In Send to Amazon, any SKU that needs an Amazon barcode shows “Print SKU labels” in its row. You can also print every unit label for the shipment with “Print all SKU labels” at the end of step 1.
- Any time from Manage Inventory. Select the products, then choose “Print item labels” from the action menu.
Both let you pick standard sheet formats or thermal printing. One small quirk: Amazon translates the condition text on the label to your Seller Central language, but not the product name.
Who puts the labels on now?
You do, or someone you pay. Amazon’s FBA Label Service page says: “Effective January 1, 2026, we’ll no longer offer prep and item labeling services.” That’s for the US store. The $0.55-per-unit option that older guides still quote only applied to shipments created before that date.
So your options are to label the units yourself, have your manufacturer apply them before shipping, or use a prep service. If you’re a reseller sending in hundreds of units a week, that’s now a line in your costs, so price it in.
🎯 Pro Insight: Ask your supplier to apply FNSKU labels before the goods ship. The labels are just PDFs, so you can send them with the purchase order. Amazon’s FBA Prep Service page says most sellers already handle labeling “on their own, through their own manufacturing partners or through third-party service providers.”
A label doesn’t stop a new seller on your listing
Ending commingling fixed one real problem: your customers now get your units, not a stranger’s. It didn’t change who can sell on your listing.
Here’s what that looks like on a busy ASIN. On October 19, 2026, a Honeywell thermostat listing we track in SellerSonar had more than 70 offers on a single ASIN. Fifteen different third-party sellers were offering it new through FBA.
In SellerSonar’s chart for that ASIN, the blue line counts every offer on the listing and the orange line counts the ones fulfilled by Amazon (about 35, including Amazon’s own offers and used ones). We started tracking it on October 16, which is why the earlier days are empty.
Since March 31, every one of them that isn’t the brand owner needs its own FNSKU labels on new shipments, so Amazon can keep their units apart. All 15 still compete for the same Buy Box, and a new seller can join the listing at any time.
That’s the part a barcode can’t help with. If you’re a brand owner, you still want to know when a new seller appears on your ASIN, and when the Buy Box changes hands. SellerSonar alerts you to both on the next scheduled check (every hour on Business and Enterprise plans; Buy Box alerts start on Premium), and our guide to Amazon listing hijacking covers what to do next.
So, do you need FNSKU labels?
Probably, unless you own the brand. Since March 31, 2026, the rules are simple even if the setup isn’t:
- Resellers (authorized or not) put an FNSKU label on every FBA unit.
- Brand owners with the brand representative role can skip stickers on new products that have a single valid UPC, EAN or ISBN and are registered to the brand.
- Labels must be 1 x 2 to 2 x 3 inches, thermal or laser printed (never inkjet), and cover every other barcode.
- Amazon won’t label for you in the US anymore, so budget for it.
Final Expert Recommendations:
If you’re a brand owner, start by checking that every product is registered to your brand in Brand Registry, then switch new offers to the manufacturer barcode. If you’re a reseller, get FNSKU labels into your supplier’s process, so nothing arrives at Amazon unlabeled and ineligible for reimbursement.
Once your units are labeled and separated, watch who else is selling on your listings. SellerSonar’s product monitoring alerts you when a new seller appears, so you can act before the Buy Box moves.
