A SKU (Stock Keeping Unit) is a unique ID for every variant of a product you stock — every color, size, scent, or version gets its own. The format isn't standardized; you design your own. Make them readable enough that a warehouse picker can spot a wrong SKU from across a shelf, and structured enough that you can sort by category or attribute when you pull inventory reports.
Get the format right early. Reworking SKUs after you've printed labels, built listings, and trained pickers is painful, so the system you set up at 50 orders a month should still hold at 5,000.
SKU vs. UPC vs. barcode
These get used interchangeably, and they shouldn't be.
- SKU: internal. You create it, you control it, it maps to how your business sorts inventory.
- UPC: external. A standardized 12-digit code assigned through GS1 that's the same everywhere the product sells.
- Barcode: the scannable image. It can encode either a SKU or a UPC. An internal SKU with letters and hyphens needs Code 128, since UPC-A holds digits only; our free SKU barcode generator renders Code 128, UPC, EAN or Code 39 labels in the browser, ready to print.
The practical takeaway: don't use a manufacturer's product code as your SKU. Two suppliers can hand you the same code, and you lose control of your own naming the day they change theirs.
How to build your SKU format
Build the SKU out of segments, each describing one attribute, ordered from broad to specific. A medium red T-shirt might be TS-RD-MED: TS for the T-shirt category, RD for red, MED for the size. A picker reads it left to right and knows what they're pulling before they see the product.
Keep three rules in mind:
- Front-load the category. Sorting an inventory report by SKU should group like products together. That only works if the broadest attribute comes first.
- Keep segment codes consistent. If red is
RDon shirts, it'sRDon hats. Mixed conventions break filtering and confuse new staff. - End with a unique identifier. A sequential number or a size/color combination guarantees no two variants ever collide.
Aim for short. A SKU detailed enough to read at a glance but short enough to type without errors beats one that encodes every attribute and takes ten seconds to scan.
SKU mistakes that cost you later
- Starting with a zero. Spreadsheets and some inventory systems drop leading zeros, so
0123silently becomes123. - Ambiguous characters. Avoid
Ovs0andIvs1. A picker misreading one is a mis-ship. - Copying another brand's system. Their attributes aren't yours. Build around how you sort and store.
- Reusing manufacturer codes. Covered above — it hands control of your catalog to someone else.
Where to find a SKU
On a physical product, the SKU is on the packaging or label, usually near the barcode. Online, it sits on the product detail page near the price or in your store admin. It also shows up on invoices, packing slips, and inside your POS — anywhere the item needs to be tracked.
Managing SKUs as you grow
A clean SKU system is what lets inventory software do its job: real-time stock counts, accurate reorder points, and sales reporting you can actually trust. Across multiple sales channels, the SKU is the key that keeps one product's stock in sync everywhere it's listed.
When order volume climbs, SKU management is one of the first things brands hand to a 3PL. At Simpl, your SKUs map to our system, we hold the inventory, and orders received before 12pm CT on a business day ship the same day; 98.8% of them did between September 21 and October 23, 2026. Pricing starts at $7/order, a flat rate per order that covers up to 3 picks, packing, postage and packaging. You keep owning the naming logic; we run the picking against it.
If you're weighing a fulfillment partner, check that they sync inventory across every channel you sell on, support real-time stock counts, and don't make you renumber your catalog to fit their system.