What is a SKU (stock keeping unit)?

What a SKU is for

A SKU answers one question: exactly which thing is this? Your stock count, reorder list and sales report all depend on that answer. A medium red shirt and a large red shirt are one product to a shopper's eye, but they sell at different rates. If they share a code, you find out you are out of mediums when a customer tells you.

Because you invent the code, you choose what goes in it. That freedom is also how lists turn messy, so it helps to settle the format before you make the first code.

SKU, UPC and the rest

SKU, UPC and the other codes on a product

Code	Who creates it	Who understands it	Does it say what the item is?
SKU	You	You and your software	Yes, if you design it to
UPC or EAN	You, from a number licensed through GS1	Every retailer and marketplace	No, it is a key for lookups
ASIN	Amazon	Amazon	No
Serial number	The maker	Anyone, for that one unit	No, and each single unit gets its own

A SKU names a kind of thing, so every medium red shirt shares one. A serial number names one single unit. A UPC code also names a kind of thing, but it does so in a form that works outside your business.

What a good SKU looks like

TSH-RED-M is easy to read: product line, color, size. Four rules cover most of what goes wrong.

1. Start with a letter. A spreadsheet reads a column of digits as numbers and drops leading zeros, so 00721 turns into 721 with no warning.
2. Keep it short. Aim for 8 to 12 characters. Every character adds width to a barcode, and a Code 128 label at a 0.33 mm narrow bar grows by 3.63 mm per character.
3. Stay with letters, digits and one separator. Spaces and symbols cause trouble when you import a list, and they can break a web address.
4. Avoid mix-ups when you can. O next to 0 and I next to 1 are the usual misreads when someone keys a code by hand.

Setting up SKUs in an afternoon

Write down the parts your business actually sorts by, most important first. A clothing seller might use product line, color and size. A parts seller might use category and a running number. Then pick one short form for each value and record it: RED always means red, and BLK always means black, on every product.

The SKU generator does the multiplying once you have your lists. Three colors and four sizes make 12 codes. It flags duplicates and leading zeros, and gives you a CSV to import. To label shelves or bins, it draws any code as a Code 128 barcode.

When you sell in more than one place

Keep the same SKU across your own store, marketplaces and spreadsheets. It becomes the link that ties your sales and stock together when each platform has its own product IDs. Check a platform's rules before you settle on a long format. eBay publishes a limit of 50 characters, Shopify advises no more than 16, and Amazon says SKUs are usually 8 to 10 letters and numbers.

A SKU never replaces a UPC where a retailer or marketplace asks for one. For that you need a licensed number, which how to get a UPC code explains.

Changing a SKU later

Avoid it where you can, because old orders and reports still use the old code. If you must change one, keep a two-column list of old and new, and do not reuse a retired code for a different product. A reused code makes your old sales history look like it belongs to the new item.

Once the codes exist, where to find a SKU number shows where each platform keeps them. The label route through Excel or Word turns a list into printed labels.