SKU

A SKU is the code you assign to one exact thing a customer can buy. Not the product in general, but the specific version of it: this shirt, in medium, in blue. The letters stand for stock keeping unit and the phrase comes from warehouses, though online it has taken on a second job that matters more, which is being the name every other system uses for that item.

In short

• One code for one exact variant, not one code per product.
• Yours to invent, unlike a barcode, which belongs to the manufacturer.
• Every connected system uses it as the item’s identity.
• Changing or reusing one silently ruins the reporting behind it.

Words you will hear

• GTIN. The global barcode number, issued through a registry and tied to the brand rather than to your shop.
• MPN. The manufacturer’s own part number, which is not a barcode and not yours either.
• Variant. One version of a product, distinguished by size, colour, weight or anything else a buyer chooses.
• Parent and child. The product as listed, and the individual variants beneath it. The child is what carries the SKU.
• Item ID. What analytics and advertising platforms call the same code once it leaves your shop.
• Stock keeping unit. The phrase in full, used mainly by people who work in the warehouse, not in marketing.

One product is usually several SKUs

The counting rule catches people out because it does not match how a shop talks about its own range.

You sell a shirt. You think of it as one product, it has one product page, and it appears once in your catalogue. If it comes in four sizes and two colours, you have eight sellable things, eight quantities to keep track of, and therefore eight codes. Whether the customer experiences that as one page with two dropdowns is irrelevant to the counting.

The same logic extends past clothing, and this is where shops most often get it wrong. A six-pack is not six of the single unit. It is its own item, with its own availability, because selling the pack removes six units and selling one removes one. A bundle of two different products is a third code, not a shortcut across the other two. Anything a customer can add to a basket as a single line needs a code of its own.

Yours, not the manufacturer’s

The distinction between a SKU and a barcode causes real confusion, and the practical consequences run in both directions.

A barcode number identifies a product globally. It is registered once by whoever makes the item, and every retailer in the world stocking that item refers to it by the same number. That is what allows advertising platforms to recognise that your listing and a competitor’s are the same object, and it is why omitting one for branded goods limits where your products can appear.

A SKU identifies the same item inside your business only. Nobody else uses it, nobody else needs to, and it can look like whatever suits you.

Which means that own-brand and handmade goods have a SKU and no barcode at all, which is entirely normal. The correct handling in a product feed is to declare that no identifier exists rather than to invent a plausible number, and the SKU carries on doing its job regardless.

Designing codes you will not regret

There is a version of this that goes wrong slowly, and it starts with a code that seemed helpfully descriptive.

Google asks that feed identifiers use letters, numbers, dashes and underscores, stay within fifty characters, and remain stable once assigned. It also suggests using your SKUs as those identifiers wherever possible, which is a reasonable default and removes a mapping nobody wants to maintain.

Within those limits, one principle does most of the work: encode only what will never change. The product and the variant qualify. Price does not, seasons do not, and neither does the supplier you happen to buy from this year. Every one of those has been baked into a code by somebody who then had to renumber a catalogue, and renumbering is exactly the operation the rest of this page is warning against.

Keep them short enough to read aloud during a stock count. Something like SHIRT-BLU-M tells a person what it is. A twenty-eight character string encoding six attributes tells them nothing and gets mistyped.

Why the code moving breaks things quietly

None of this would matter much if a wrong SKU produced an error. It does not. It produces a number that looks fine.

Analytics records the identifier against every view, every add to basket and every purchase. Advertising platforms use it to match a visitor to the exact item they looked at, which is what makes retargeting show the right product. Marketplaces hold it against your listings. Your e-commerce platform holds it against your stock.

So renaming a code splits that item’s history into a before and an after, with nothing joining them, and the platform treats the new code as a product it has never seen. Reusing an old code does the reverse and merges two products into one line. Neither event raises a warning anywhere, and both are usually discovered months later by somebody trying to work out why a best seller appears to have launched in March.

Sellers running their own shop alongside Shopee and Lazada meet a version of this almost immediately, because each marketplace will happily assign its own code. The habit worth forming early is to keep one master SKU in your own system and map everything else to it, rather than letting three platforms each hold a different name for the same box on the same shelf.