If you sell on Amazon and you have stared at a Seller Central column labelled FNSKU wondering whether it is the same thing as the UPC printed on your carton, this is for you. By the end you will know what each identifier does, who issues it, which one belongs on the physical unit, and which one will get your shipment flagged. The FNSKU vs UPC question costs sellers more money than any other labelling question, and the rules changed in March 2026, so we start there and work outward.
Three acronyms first. FBA is Fulfillment by Amazon. A 3PL is a third-party logistics provider, a warehouse that stores and ships your inventory for you. An FC is a fulfillment centre, Amazon’s own warehouse.
The three identifiers in one table
| Identifier | What it identifies | Who issues it | Where it physically lives |
|---|---|---|---|
| UPC (Universal Product Code) | The product | GS1 US, licensed to your company | Printed on your retail packaging |
| ASIN (Amazon Standard Identification Number) | The listing, meaning one detail page in Amazon’s catalogue | Amazon | In Amazon’s catalogue and in the product page URL. Not normally printed by you |
| FNSKU (Fulfillment Network Stock Keeping Unit) | Your units of that product, inside Amazon’s network | Amazon, one per seller per SKU | On a label applied to each unit before it ships to an FC |
Read that table twice. The UPC says what the thing is. The ASIN says which page it sells on. The FNSKU says whose unit this is. Everything else follows from that.
What a UPC is, and why Amazon checks where you got it
A UPC is a twelve-digit number. In GS1’s naming it is a GTIN-12, where GTIN means Global Trade Item Number. It is built from a GS1 company prefix, an item reference number, and a check digit. GS1 US licenses the company prefix to your business, and that licence is what ties the number to your brand in GS1’s database. Amazon validates against that database, which is why cheap resold barcodes cause listing problems.
Every variant needs its own UPC. Three sizes and two colours is six UPCs, not one.
The thing founders miss: a UPC identifies the product, not you. If four companies resell the same bottle of shampoo, all four units carry the same UPC. Nothing on that barcode says which of the four sellers owns a given bottle. That single fact is the reason FNSKUs exist, and it is the reason we include FNSKU labelling in our FBA prep services rather than treating it as an optional extra.
What an ASIN is
An ASIN is Amazon’s internal key for a catalogue listing. Ten characters, usually starting with B0 for non-book products. One detail page, one ASIN, however many sellers are competing on it.
Variants get their own ASINs. The small black tee and the large black tee are two ASINs sitting under one parent listing.
You do not print ASINs on your packaging as a matter of course. There is one exception, covered below, where the ASIN shows up in your FNSKU field and ends up on the label.
What an FNSKU is
An FNSKU is the barcode Amazon issues to tie a specific unit to your seller account inside the fulfillment network. It is ten characters and it starts with X00. That is X, zero, zero, not X, letter O, letter O. Sellers get this wrong when they hand-key it, and a mistyped prefix means a label that scans to nothing.
You generate FNSKU labels in Seller Central, print them, and apply one to every unit. The label covers any other scannable barcode on the package.
One FNSKU per seller, per SKU. Change the SKU and Amazon issues a new FNSKU, which is how sellers separate expiry-dated lots from each other.
FNSKU vs UPC: which barcode actually goes on the unit
This is the decision, and since March 2026 it depends almost entirely on whether you own the brand.
| Your situation | Goes on the unit | Why |
|---|---|---|
| Brand Registry brand owner, product meets eligibility | Manufacturer barcode (UPC or EAN), no sticker | Tracked virtually by Amazon |
| Brand owner, product fails eligibility (expiry-dated, topical, media) | FNSKU label | Virtual tracking not available |
| Reseller, wholesale or arbitrage, any product | FNSKU label | Required since 31 March 2026, even with a valid UPC |
| No manufacturer barcode at all, GTIN exemption granted | FNSKU label | There is nothing else to scan |
| Multi-unit bundle you assembled | One FNSKU for the set | The set is its own SKU. Component barcodes get covered |
Amazon’s own help page states the principle plainly: Amazon barcodes must be applied to every product that is not being tracked by the manufacturer barcode.
What changed on 31 March 2026
Amazon ended inventory commingling and split barcode rules by seller type. In Amazon’s wording on Seller Central, brand owners can keep using manufacturer barcodes without stickers, while resellers are now required to use Amazon barcodes even when the product already carries a manufacturer barcode.
If you buy wholesale and resell other people’s brands, you label every unit. There is no longer a stickerless route for you. If you own your brand and you are enrolled in Brand Registry as the brand representative, you may be able to stop labelling entirely.
That is the shorter version of a change that caught a lot of sellers mid-quarter. We wrote up what sellers have actually done since Amazon stopped prepping inventory itself in January 2026, and the barcode change landed on top of that one.
The eligibility criteria, in Amazon’s own terms
Virtual tracking with a manufacturer barcode has never been open to every product. Amazon has published these conditions: the item must be in new condition, carry a single scannable UPC, EAN or ISBN matching one ASIN, and must not be an FBA-restricted product or dangerous good, an expiry-dated product, a consumable or topical product such as groceries, skin creams, cosmetics or soaps, or a media product such as books, CDs or DVDs.
Skincare, supplements and food are the categories that trip founders up. If you sell any of those, you are labelling, brand owner or not.
Where a product fails the eligibility test, Amazon points brand owners toward an exemption route through Brand Registry enrolment, while sellers who are not brand-registered must apply Amazon barcodes to the ineligible product.
Check eligibility per listing in Seller Central before you order 40,000 labels. Amazon updated these criteria alongside the March change and decides eligibility at the listing level, not the category level.
What a compliant label looks like
Three quarters of the labelling problems we see at the dock are print problems, not decision problems.
Amazon’s guidance to sellers printing their own labels: use blank mailing labels at least 1 to 2 inches tall and 2 to 3 inches wide, black ink on non-reflective adhesive paper, laser printer, 300 DPI. The label carries the scannable barcode, the number underneath it, the product title and the condition, stated as new or used.
Leave white space around the barcode: a quarter inch at the sides, an eighth at top and bottom. Keep a quarter inch between the label edge and the package edge. Do not put a label on a curve or a corner, which is why cylindrical bottles and tubes fail scans more than boxes do.
Put the correct barcode on each item and cover every other visible barcode, with two exceptions: serial number barcodes and Transparency authentication labels stay visible. Photocopied labels are rejected.
Where this goes wrong on our floor
We receive FBA-bound inventory at four Southern California warehouses, in Jurupa Valley, Lake Forest, Commerce and Perris, which puts us close to the Inland Empire FCs most SoCal sellers route into: ONT2, ONT8 and LAX9. The same four failures come across the dock:
The supplier printed the UPC on the box and the seller applied the FNSKU to the poly bag. Two scannable barcodes, one unit, flagged.
The label file was generated for last quarter’s SKU. The SKU changed, the FNSKU changed, the printer did not.
The label went on the shoulder of a bottle, where the curve breaks the bars.
A reseller shipped stickerless out of habit, because that is what worked for three years before March.
None of those are complicated. They are all cheap to prevent at the point of labelling and expensive to fix once a pallet is at an FC. If you want your Amazon orders flowing without a spreadsheet in the middle, our Amazon FBA integration connects directly through the API rather than through file uploads.
When you should not pay anyone to label your units
We will say this plainly because it costs us money to say it.
If you are a Brand Registry brand owner and your products pass eligibility, you do not need labels, and you should not be paying a prep centre per unit to apply them. Ask your current provider whether your SKUs are eligible for virtual tracking. At ten cents a unit, 20,000 units a year is $2,000 spent on stickers Amazon never asked for. That is an easy phone call and it is worth making before you sign anything with us or anyone else.
If you ship under 50 orders a month and you are not growing toward more, buy a thermal label printer, print from Seller Central, and do it at your kitchen table. Outsourcing at that volume costs more than it saves. Our own invoice minimum is $500 a month, stated up front, which is the wrong shape of cost for a seller moving 200 units a quarter. We publish what fulfillment actually costs so you can run that maths before you call us.
If labelling is your only problem and you already have warehouse space, a labelling-only vendor is cheaper than moving your whole operation to a 3PL. Moving fulfillment is worth doing when storage, picking, shipping and returns are all straining at once, not when one task is annoying.
We are also the wrong answer for cold-chain and hazmat inventory, and for brands whose customers are overwhelmingly on the East Coast and need same-day delivery there.
Common questions
Is the FNSKU the same as my SKU?
No. Your MSKU, the merchant SKU, is the code you invented for your own records. The FNSKU is Amazon’s code for your units of that listing inside its network. They sit in different columns and they are not interchangeable.
Do I still need a UPC if I am using FNSKU labels?
Yes, in almost every case. The UPC is how you create the listing in the first place. The FNSKU is how Amazon tracks units afterward. The exception is a granted GTIN exemption, where Amazon lets you list without a manufacturer barcode and the Amazon barcode does all the work.
Can I leave both barcodes showing?
No. Cover the manufacturer barcode when an FNSKU label is required. Two scannable codes on one unit is a scan ambiguity, and the FC resolves ambiguity in its own favour, not yours.
Why does my FNSKU look like an ASIN?
On listings tracked virtually with the manufacturer barcode, Seller Central often shows the ASIN sitting in the FNSKU field. That is normal, and it is a signal that the SKU is not expecting a sticker. Confirm it rather than assuming it.
What to do next
Open Seller Central, go to your FBA inventory, and sort by FNSKU. Any SKU showing an X00 code needs a label on every unit. Any SKU showing a B0 code is being tracked virtually and does not. That takes ten minutes and it tells you exactly what your prep bill should look like.
If the answer is that you are labelling thousands of units a quarter and doing it in a garage, that is the point where a Southern California prep operation starts paying for itself. You can see the rest of what we run from our SoCal floors, or book a 15-minute call and we will size it against your actual SKU count and volume. You will speak to a person, not a ticket queue.