An Amazon SKU is a merchant-defined identifier that a seller assigns to every unique product variant they list on Amazon. Unlike ASIN, which Amazon controls, the SKU is created and owned by the seller and can follow any naming convention. It ties a Seller Central listing to the seller’s internal inventory, purchase orders, and accounting records, so the same physical product often has one ASIN but many SKUs across different sellers.
Why It Matters for Sellers
The SKU is the anchor that connects Amazon inventory to a seller’s back office. Every FBA shipment, replenishment report, storage fee, and profit calculation is grouped by SKU. A messy SKU scheme — reused SKUs across variants, or SKUs that do not encode brand, size, color, and pack quantity — makes reconciliation painful and can lead to stranded inventory when Amazon’s catalog and the seller’s ERP disagree. A disciplined SKU convention also makes it easy to spot duplicated listings, retire dead variants, and audit margin at the variant level.
How to Structure a SKU
Amazon allows any alphanumeric string up to 40 characters, but the useful patterns share a few traits:
- Encode brand or supplier code in the first segment (e.g., ACME).
- Encode product family and variant attributes (color, size, pack) in short codes.
- Keep a permanent, non-reusable serial — never recycle a retired SKU on a new product.
- Avoid spaces and special characters that break flat files or CSV uploads.
Example: `ACME-TSHIRT-BLK-L-2PK` is easier to audit than `sku0042`.
SKU vs ASIN vs FNSKU
| Identifier | Assigned By | Scope | Format |
|---|---|---|---|
| SKU | Seller | Per seller, per variant | Any alphanumeric, up to 40 chars |
| ASIN | Amazon | One per product across the whole catalog | 10-character alphanumeric |
| FNSKU | Amazon (FBA only) | Per seller, per ASIN, inside FBA | Starts with X, 10 chars total |
The same t-shirt has one ASIN, but ten sellers offering it have ten different SKUs. When that t-shirt enters FBA, Amazon issues a per-seller FNSKU that gets printed on the physical unit label.
How SellerSonar Helps
SellerSonar Product Monitoring watches every ASIN in your catalog, and Retail Issues Alerts surface SKU-level catalog mismatches that cause suppressions.
Start your free SellerSonar trial today.
Related Terms
- ASIN — Amazon’s internal 10-character product ID
- FNSKU — the FBA-specific unit label Amazon assigns per seller
- Amazon FBA — the fulfillment program that generates FNSKUs from your SKUs