What Is a SKU? A Complete Guide for Online Sellers (With Examples)

What is a SKU?

SKU stands for Stock Keeping Unit. It is a unique alphanumeric code you assign to each distinct product — or product variant — in your inventory. SKUs are internal identifiers, meaning you create them yourself using any format you choose. Unlike a barcode, there is no external authority issuing them.

SKUs serve one core purpose: to give every distinct item in your inventory an unambiguous, unique identity that your systems can track. Every time a unit of a product is sold, received, transferred, or adjusted, the transaction is recorded against its SKU. Without SKUs, you are essentially tracking inventory by product name — which breaks the moment you have two products with similar names, or the same product listed differently across platforms.

Here are some examples of what good SKUs look like:

• TSHIRT-BLK-S — black T-shirt, small
• TSHIRT-BLK-M — black T-shirt, medium
• TSHIRT-BLK-L — black T-shirt, large
• MUG-WHT-11OZ — white ceramic mug, 11oz
• NOTEBOOK-A5-LN — A5 notebook, lined
• CABLE-USB-C-1M — USB-C cable, 1 metre

Each SKU encodes the product category, key identifying attributes, and — for variants — the specific variation. Anyone looking at the SKU can immediately understand what product it refers to, which makes your operation more legible to staff, suppliers, and any tools you integrate.

SKU vs barcode vs UPC vs ASIN vs FNSKU — what's the difference?

New sellers often confuse these terms. They are all product identifiers, but they serve completely different purposes. Here is a clear comparison:

The practical takeaway: your SKU is yours to create and control.EAN barcodes are standardised and used for shipping and retail scanning. ASINs and FNSKUs are Amazon-specific and assigned by Amazon. Your SKU is the identifier that ties all of these together in your internal system — it's the thread that lets you match "TSHIRT-BLK-M on Shopify" with "TSHIRT-BLK-M on eBay" and "TSHIRT-BLK-M in your Amazon FBA inventory".

How to create a SKU naming system

A good SKU system is readable, consistent, scalable, and unique. Here is a practical approach for UK ecommerce sellers.

Step 1: Choose a structure

The most common and workable structure for product SKUs is:

CATEGORY-PRODUCT-VARIANT

Where each segment is a short abbreviation. For example:

• APP-HOODIE-NVY-M — Apparel, Hoodie, Navy, Medium
• APP-HOODIE-NVY-L — Apparel, Hoodie, Navy, Large
• APP-HOODIE-GRN-M — Apparel, Hoodie, Green, Medium
• HOME-CANDLE-LAV-200G — Home, Candle, Lavender, 200g
• ELEC-CABLE-USBC-1M — Electronics, Cable, USB-C, 1 metre

Step 2: Decide your abbreviations

Write down a list of your product categories and the most common attributes (colour, size, scent, material, etc.) and create consistent short codes for each. Keep a spreadsheet of these codes so you and your team always use the same abbreviations.

Colour abbreviations used by many UK sellers:

• BLK — Black
• WHT — White
• NVY — Navy
• RED — Red
• GRY — Grey
• GRN — Green
• BLU — Blue
• PNK — Pink
• MLT — Multicolour

Size abbreviations:

• XS, S, M, L, XL, XXL — for clothing
• SML, MED, LRG — for non-clothing size variants
• Numeric where relevant (e.g. 200G, 500ML, 1M, 2M)

Step 3: Apply rules for uniqueness

Every SKU must be unique. The simplest way to guarantee this is to generate them systematically from your master list and never reuse a SKU — even if a product is discontinued. If you ever deactivate a product, retire its SKU rather than reassigning it to something else. Retired SKUs in old order records need to mean the same thing they always did.

Step 4: Keep them short and uppercase

SKUs do not need to be human-readable sentences. Shorter is better — aim for under 20 characters. Always use uppercase. Use hyphens to separate segments, not spaces or special characters. Spaces in SKUs cause problems on some platforms (particularly in CSV imports and API calls).

SKU best practices for UK sellers

Never use the supplier's code as your SKU

It's tempting to use a supplier's product code as your SKU — it's already there, it's unique, and it saves effort. But supplier codes are designed for the supplier's system, not yours. They often contain characters that cause problems (slashes, dots, mixed case). More importantly, if you switch suppliers for the same product, your internal records get inconsistent. Use supplier codes as a reference field, not as your SKU.

Give every variant its own SKU

A red medium T-shirt and a blue large T-shirt are different inventory items. They need different SKUs. If you track them under the same SKU, you cannot tell how many of each variant you have. This sounds obvious, but it is one of the most common errors made during catalogue setup — particularly when importing products in bulk from a spreadsheet where variants might be collapsed together.

Record your SKU master list centrally

Keep a master spreadsheet (or use your inventory software) that records every SKU along with the product name, description, category, and supplier. This becomes your product catalogue reference and makes onboarding new staff or new selling channels much faster.

What happens without a consistent SKU system

The problems that emerge from poor SKU management are predictable and expensive:

• Overselling. Without unique SKUs, your inventory software cannot match listings across channels. Stock syncs fail silently and you end up selling stock you don't have.
• Wrong stock adjustments. If two similar products share a SKU or have confusingly similar names, manual stock updates get applied to the wrong item.
• Reporting becomes unreliable. Sales reports, margin calculations, and reorder triggers all depend on SKUs being correct. Bad SKUs mean bad data throughout.
• Amazon complications. Amazon uses ASINs and FNSKUs but also expects seller SKUs to be consistent. FBA inventory management becomes extremely difficult without reliable SKUs on your side.
• Team errors multiply. When anyone on your team — or a VA, or a fulfilment partner — handles products without clear SKU reference points, picking errors increase.

Fixing a broken SKU system after the fact is painful. The time to get it right is before you list your first product across multiple channels.

SKUs across multiple selling channels

The single most important principle for multi-channel sellers is this: use the same SKU for the same product on every platform.

When you list TSHIRT-BLK-M on Shopify, eBay, and Amazon, all three listings should have the SKU TSHIRT-BLK-M. This is how your inventory software knows that a sale on eBay should reduce the stock count for the same item on Shopify and Amazon.

Each channel has its own field for seller SKUs:

• Shopify — SKU field at the variant level
• eBay — Custom Label field in your listing (in Seller Hub, this is labelled "Custom label (SKU)")
• Amazon — Seller SKU field (separate from the ASIN, which Amazon assigns)
• Etsy — SKU field in each listing variation
• WooCommerce — SKU field at the product or variation level

If you have existing listings without SKUs or with inconsistent SKUs, the cleanest fix is to go through them systematically and update each one. This is tedious but worth doing before you connect any inventory sync tool — otherwise the tool will simply not be able to match your listings.

SKUs for product variants

Variants are products that share a parent listing but differ in one or more attributes — size, colour, material, scent. Each variant needs its own SKU.

The recommended approach is to use a base SKU for the parent product and append the variant attributes:

• Parent product: Heavyweight Cotton T-Shirt
• Variant SKUs: TSHIRT-HVY-WHT-S, TSHIRT-HVY-WHT-M, TSHIRT-HVY-WHT-L, TSHIRT-HVY-BLK-S, TSHIRT-HVY-BLK-M, etc.

This structure makes it immediately clear what the item is and which variant it is, and the shared prefix (TSHIRT-HVY) groups them together in sorted lists and reports.

On eBay, multi-variation listings group variants under a single listing with separate Custom Labels per variation. On Amazon, variants are typically grouped under a parent ASIN with child ASINs for each variation — your Seller SKU goes at the child ASIN level.

How Salync uses SKUs to link listings across channels

Salync uses your SKUs as the primary matching key when connecting your channels. When you connect Shopify and eBay to Salync, it reads the SKU from every listing on both platforms and builds a map of which listings represent the same physical product.

When a sale happens on eBay for TSHIRT-BLK-M, Salync:

1. Detects the sale via the eBay API
2. Looks up TSHIRT-BLK-M in its channel map
3. Finds all matching listings across Shopify, Amazon, Etsy, etc.
4. Deducts the quantity sold from the central stock count
5. Pushes the updated quantity to every matching listing within seconds

If a listing has no SKU, or has an SKU that doesn't match anything on another channel, Salync flags it as unmatched. You can then either assign a SKU or manually link the listings. The matching is transparent — you can always see which listings are linked and why.

Common SKU mistakes

1. Using spaces in SKUs

T Shirt Black M looks readable but breaks in CSV imports, URL encoding, and some platform APIs. Always use hyphens: TSHIRT-BLK-M.

2. Mixing case inconsistently

TShirt-Blk-m, TSHIRT-BLK-M, and tshirt-blk-m are three different strings. Pick a case (uppercase is standard) and stick to it everywhere.

3. Reusing SKUs from discontinued products

A retired product's SKU must never be reassigned to a new product. Old order records reference it. Historical reports reference it. Reusing it causes confusion and data corruption.

4. Creating new SKUs spontaneously without a master list

If multiple people in your business create SKUs ad hoc, you will end up with duplicates, inconsistent formats, and abbreviations that mean different things. Maintain a master SKU list and make sure everyone follows it.

5. Treating the ASIN as your SKU

Amazon's ASIN is not your SKU — it is Amazon's identifier for a product in their catalogue. Amazon expects you to provide a Seller SKU separately. Use your internal SKU as the Seller SKU so everything stays consistent.

6. Not applying SKUs to Etsy listings

Many Etsy sellers skip the SKU field because Etsy doesn't require it. But without SKUs on your Etsy listings, you cannot sync Etsy stock with other channels. Add SKUs to every Etsy listing and variation.