What is a SKU? Stock keeping units explained, with examples
A SKU (stock keeping unit) is a code a seller creates to identify each distinct item it stocks, down to the size, color or pack size, so it can count, sell and reorder that exact item. Schema.org defines it as "a merchant-specific identifier for a product or service."
SKU vs UPC, GTIN and MPN
A SKU is internal: you invent it and it only has to be unique in your own catalog. Other identifiers are shared across the supply chain.
| Identifier | Who creates it | Scope |
|---|---|---|
| SKU | The seller (brand or retailer) | Unique within one business |
| GTIN (UPC, EAN) | The brand owner, using a company prefix assigned by GS1 | Global; a UPC-A barcode carries 12 digits |
| MPN | The manufacturer | That manufacturer's part number |
As GS1 US puts it, "Two retailers can sell the same item with the same UPC codes, but they will both use their own SKU number."
How to build a SKU
A common approach is a short, readable pattern made of fixed segments, most general first:
- Product family or category: TS (T-shirt)
- Model or style: CREW
- Variant attributes: BLK (black), M (medium)
Example (hypothetical): TS-CREW-BLK-M. The same shirt in white, large becomes TS-CREW-WHT-L. Each size and color combination is a separate variant and gets its own SKU, because each has its own stock count. A shirt offered in 4 colors and 5 sizes therefore needs 20 SKUs.
Rules that keep SKUs clean
- Use letters, digits and one separator; avoid spaces and special characters.
- Keep a fixed order and length for each segment so codes sort and filter well.
- Avoid characters that are easy to misread, such as O and 0 or I and 1.
- Don't start with zeros; spreadsheets often strip them.
- Don't encode things that change, such as price, supplier or season.
Why SKUs matter for an online store
- Accurate inventory. Stock, reorder points and sales reports all key off the SKU.
- Fulfillment. Pickers, packing slips and returns refer to one exact item.
- Channel feeds. Google Merchant Center recommends using your product SKU as each product's id, which must be 1 to 50 characters, unique for each product and variant, and never changed: changing it overwrites the product and its history.
- Search results. The sku property in product structured data helps search engines tell your items apart.
Common mistakes
- Reusing a SKU. Giving a discontinued code to a new product mixes their sales history. Google also asks sellers not to recycle IDs.
- Relying on case. Google says not to make IDs differ only by upper and lower case; ab12 and AB12 should not be two products.
- One SKU for many variants. If every size shares a code, you cannot tell which size is running out.
- Using the barcode as the SKU. It can work, but a GTIN belongs to the brand owner, and items you bundle or make yourself may not have one.
Quick answers
Is a SKU the same as a product ID?
Often, yes: many platforms use the SKU as the product or variant identifier in feeds and reports.
Do I need a barcode to have a SKU?
No. You can create SKUs yourself; a GTIN is built on a company prefix that GS1 assigns to the brand owner.