What is a SKU and how to use them in your store

A SKU is not a barcode, not a manufacturer's code, and not a customer-facing reference number. It is an internal identifier that you define for your own operational purposes. Getting that distinction right from the start prevents a lot of confusion about what SKUs are for and how they should be structured.

What is a SKU?

A SKU, short for stock keeping unit, is a unique alphanumeric code assigned to a specific product variant in your catalog. It identifies one specific combination of attributes, such as product type, color, and size, within your inventory system. When a buyer orders a blue hoodie in size large, the SKU for that exact variant is what your system uses to locate it, deduct it from stock, and record the sale.

SKUs are internal codes. Buyers rarely see them. They are designed to be read by your system, your warehouse team, and your suppliers, not by your customers. This means they can be formatted however serves your operations best, without worrying about how they look on a product page.

Every variant of a product needs its own SKU. A t-shirt in three colors and four sizes has twelve variants and therefore needs twelve SKUs. Assigning a single SKU to the whole product and relying on a separate size and color note is how inventory systems lose accuracy. The SKU is what makes each specific version of a product trackable as an independent unit.

How is a SKU different from a barcode or UPC?

A barcode, specifically a UPC (Universal Product Code), is a standardized identifier assigned by an external registry. It is used to identify products at retail, in supply chains, and on marketplaces. Barcodes are universal, meaning a barcode scanner anywhere in the world reads the same number for the same product. You do not create them yourself. You apply for them through a registry organization.

A SKU is something you create yourself, for your own internal use. Two different stores selling the same product will likely use different SKUs for it. A SKU has no universal meaning outside your organization. It is a code that makes sense within your catalog structure and your fulfillment process.

You can use both. A product can have a manufacturer's barcode for marketplace selling and supply chain integration, and a SKU that fits your internal naming structure. The two codes serve different purposes and live in different systems. On your store, the SKU is what your inventory management tracks. On a marketplace or in a warehouse with barcode scanning, the UPC takes over.

Why does a good SKU system matter?

Look at any store that manages inventory without structured SKUs and you will find the same problems. A blue shirt in size medium and a blue shirt in size large both described as "Blue Shirt" in a spreadsheet. A fulfillment team that has to read a written description for every pick. A reorder process that identifies the wrong product because two items have similar names but no distinct code.

A structured SKU system eliminates description-based errors. When every variant has a unique, readable code, the room for misidentification shrinks dramatically. Your fulfillment team picks by code, not by description. Your inventory system deducts by code, not by a text match. Your reorder reports list specific codes, not product names that might be ambiguous.

As a catalog grows, this becomes more significant. With 20 products, you might manage without SKUs by relying on careful descriptions. With 200 products and multiple variants each, the system without structured SKUs will fail regularly. Building a SKU structure early, while your catalog is still manageable, is far easier than retrofitting one after inventory errors have already accumulated.

How do you build a SKU structure?

A SKU is typically built from a sequence of codes representing the product's key attributes, separated by dashes or another delimiter. The goal is to make the code readable at a glance by anyone working with your inventory, without having to look it up.

Choose your attributes in order

Decide which attributes define a variant and in what order they appear in the SKU. A common structure for an apparel store is product category, then product name or code, then color, then size. So a blue medium hoodie in the "Maple" design from a brand's hoodies range might be HUD-MAP-BLU-M. Anyone handling that order knows it is a hoodie, Maple design, blue, medium, without reading a description.

Keep the attribute order consistent across your entire catalog. If color comes before size in one SKU, it must come before size in all SKUs. Inconsistency defeats the point of having a structured system. The code becomes unreadable if the order of segments varies.

Use short, fixed-length codes for each segment

Each segment of a SKU should be a short, consistent abbreviation. Three characters is a common standard for each segment. BLU for blue, MED for medium, HOD for hoodie. Avoid abbreviations that look identical for different values. BLK for black and BLU for blue are distinct. BL for both is not. Test your abbreviation choices against your full range of values before committing to them.

Avoid special characters and spaces

SKUs should contain only letters and numbers, with dashes or underscores as delimiters if you need separators. Spaces, slashes, and special characters cause problems in spreadsheets, inventory systems, and when SKUs are exported or imported between tools. A SKU like HOD-MAP-BLU-M works everywhere. A SKU like HOD / MAP BLU (M) will break at least one system it passes through.

Do not embed supplier codes into your SKUs

Using a supplier's own product code as your SKU creates a dependency on that supplier's naming system. If you switch suppliers, or if the supplier changes their code, your SKU system breaks. Your SKUs should reflect your catalog attributes, not your supplier's catalog. Keep supplier references separate, as a supplier code field that lives alongside your SKU rather than inside it.

How many characters should a SKU be?

There is no universal rule, but shorter is generally better. SKUs between 8 and 20 characters are workable for most catalogs. Longer SKUs are harder to read, harder to type correctly, and harder to scan at a glance during picking. If your attribute sequence produces a SKU of 30 or more characters, look for segments that can be abbreviated more tightly or removed without losing uniqueness.

The SKU only needs to be unique within your catalog, not universally unique. You do not need to add random characters or lengthy prefixes to guarantee uniqueness globally. A structured code built from your own catalog attributes will be unique across your products as long as the attributes themselves are distinct.

What should you do when a product is discontinued?

When a product is discontinued, its SKU should be retired rather than reassigned. Never reuse a SKU for a different product. Historical order records, inventory reports, and supplier correspondence that reference the old SKU will become confusing or inaccurate if the same code now refers to something different.

Mark retired SKUs as inactive in your system. Keep them visible in your records for as long as your historical data is relevant, typically a minimum of three years for tax and accounting purposes. Do not delete them. An inactive SKU that still appears in your historical reports is useful information. A deleted SKU that appears in old order records as an unresolvable reference is a headache.

How do SKUs connect to inventory management?

SKUs are the foundation that makes inventory tracking functional at the variant level. Without a SKU per variant, your inventory system can tell you how many units of a product you have in total. With a SKU per variant, it can tell you how many blue medium hoodies you have. That specificity is what makes reorder decisions accurate and stockout prevention possible.

Every reorder point, every low stock alert, and every inventory count in a well-managed store operates at the SKU level. When you set a reorder point of 10 units, you set it for a specific SKU, not for a product as a whole. When your system sends a low stock notification, it references the SKU so you know exactly which variant needs attention. For a full breakdown of how inventory tracking and reorder points work in practice, see how to manage inventory for your online store.

How do SKUs help with picking and fulfillment?

In a fulfillment context, a SKU is how the correct item gets located and pulled for an order. A pick list generated from an order typically includes the SKU, the quantity, and the storage location for each item. A picker working from a SKU is working from a code that maps to exactly one product variant. There is no ambiguity in the way there would be if the pick list said "Blue Hoodie, Medium" and two similar products occupied the same area.

For stores fulfilling from home or a small workspace, SKUs allow you to create a simple location system. Each SKU has a designated storage spot. When an order comes in, you pull the SKU referenced on the order from its spot. No searching, no guessing, no picking the wrong variant because two products look similar on the shelf.

Common SKU mistakes to avoid

Using the product name as the SKU

Product names are too long, too variable, and too subject to change to serve as SKUs. If you rename a product or refine its description, the "SKU" changes too, which breaks every historical record that references the old name. SKUs are codes, not labels.

Starting SKUs with a zero

Leading zeros in SKUs create problems when SKU data passes through spreadsheets, which often strip leading zeros from numeric fields. A SKU of 00142 becomes 142 in a spreadsheet, which no longer matches the original code. Use letters or non-zero digits at the start of every SKU.

Creating SKUs that are too similar to each other

SKUs that differ by only one character are easy to confuse during manual data entry or fulfillment. If BLU-M and BLK-M look nearly identical in a picking context, errors will happen. Build enough differentiation into your abbreviation choices that adjacent SKUs are clearly distinguishable under time pressure.

How WEMASY supports SKU management

WEMASY's e-commerce system lets you assign a SKU to each product and each variant individually, track stock at the SKU level, and set reorder thresholds per SKU so low stock alerts fire for specific variants rather than just at the product level. SKUs are included in order exports and inventory reports so your fulfillment records stay organized. See what is included in each plan on the pricing page. For more on how your catalog is structured and organized, see how to organize your product catalog with categories and tags.

Frequently asked questions