Attach Rate

Attach rate is the share of transactions in a host category that also include a given accessory or add-on item — the belt that sells with the denim, the care kit that sells with the boot, the brush that sells with the foundation. It measures demand that is generated by another product's traffic rather than by the accessory's own, which is why it is the planning metric for any item whose sales follow a host: accessories and bags, footwear care and add-ons, beauty tools and applicators, and the attach layer inside an apparel assortment.

The distinction from units per transaction matters. UPT counts every item in the basket and belongs to the whole store. Attach rate names a host and an attached item, and asks one question: when the host sells, how often does this item go with it? That framing is what makes it usable for allocation, because it ties the accessory's demand to a decision the brand has already made — where the host was placed.

How it is computed

Attach rate = transactions containing both the host and the attached item ÷ transactions containing the host. The denominator is host transactions, not total transactions and not host units, and the period and the location have to match on both sides. It can be computed per door, per channel or per cluster, and the door-level figure is the one that changes allocation, because attach behaviour differs by door as much as size behaviour does.

The figures below are illustrative, chosen because they divide cleanly; they are not benchmarks and are not drawn from any brand. A door records 400 transactions containing denim in a period, and 60 of those transactions also contain a belt. The belt's attach rate to denim in that door is 60 ÷ 400 = 0.15, or 15 per cent. A second door is about to receive an allocation that will support 1,000 denim transactions over the same length of period. If its attach behaviour matches the first door's, the belt demand generated by that denim is 1,000 × 0.15 = 150 units — before a single belt has been sold there, and regardless of what the belt's own sales history in that door says.

Use in allocation and assortment

The allocation use follows directly. An attached item is placed in proportion to the host's allocation, not to its own history, because its history is a record of where it happened to be placed last time rather than of where its demand is. The door that received the denim depth gets the belts; the door whose host allocation was cut loses the attached depth with it. This is the attach-rate-driven placement described in the allocation by vertical guide, and it is the placement a category-level sales history would never propose.

The assortment use is the mirror. A low attach rate on an item designed to attach — a bag colorway that does not sell with the coat it was built for — is evidence against the option, independent of its total units, and a high attach rate on an item that was not designed to attach is evidence for a placement change. Attach rate also decides replenishment source: attached items with stable rates run on the host's replenishment rhythm rather than on their own trigger, which is how an evergreen core accessory is kept in stock alongside the products that sell it.

RetailNorthstar holds door-level sell-through and allocation in one connected workflow, so an attached item's placement can follow the host's rather than being rebuilt from the accessory's own history each season. See how RetailNorthstar handles allocation →