What attach rate means
Attach rate is the share of transactions containing an anchor item that also contain a second, specified item. If 1,000 baskets include a coffee and 220 of them also include a breakfast sandwich, the sandwich attaches to coffee at 22%.
The formula is one line:
Attach rate = baskets with anchor and attachment / baskets with anchor
The denominator is the discipline. It is not all transactions, it is only the transactions that had the chance to attach. A store selling 220 sandwiches out of 4,000 total baskets has a 5.5% penetration of the whole store and a 22% attach rate to coffee, and only the second number tells you anything about whether the sandwich is being merchandised well.
Why it drives money faster than traffic does
Because the traffic is already paid for. A store that grows attach rate is earning more from customers who were walking in anyway, with no additional marketing, no additional labour and no additional rent. That makes attach rate the cheapest revenue line on the page.
Work it through for Sunrise Market, an illustrative operator, over one week:
| Scenario | Coffee baskets | Attach rate | Sandwiches sold |
|---|---|---|---|
| Current, one week | 1,226 | 22% | 270 |
| Plus 4 points, one week | 1,226 | 26% | 319 |
| Current, 52 weeks | 63,752 | 22% | 14,025 |
| Plus 4 points, 52 weeks | 63,752 | 26% | 16,576 |
Four points of attach rate on an unchanged customer count is 2,551 additional sandwiches a year from a single site. Growing traffic by the equivalent amount would mean finding thousands of new visits. This asymmetry is why foodservice programmes are built around attachment rather than around destination traffic.
Where the measurement goes wrong
Choosing an anchor nobody buys. Attach rate against a low-volume anchor is a ratio of two small numbers and swings wildly. Anchor on the item that actually drives visits.
Confusing attach rate with basket penetration. Penetration divides by all baskets and answers a different question. Both are useful; reporting one under the other's name is common and wrecks comparisons.
Ignoring the daypart. A 22% weekly attach rate might be 31% in the morning and 9% in the afternoon. The average describes no actual hour of the business, and the fix for each block is different.
Treating correlation as causation. Coffee and sandwiches co-occur partly because breakfast is one occasion, not because the coffee caused the sandwich. Attach rate measures the pattern; whether merchandising moves it is a question you answer by changing something and watching.
Attach rate in a convenience context
Convenience is close to the ideal setting for the metric. Baskets are small, usually one to three items, so a second item is a large proportional change to the transaction. The occasion is time-bound, so the attachment decision happens in seconds at the point of purchase. And the highest-margin categories are exactly the attachable ones: NACS reports foodservice contributing 38.9% of convenience in-store gross profit dollars in 2025 from 28.5% of sales, so foodservice items earn more per dollar than the store average.
That gap is the entire argument for attach rate as an operating metric in this channel. Attaching a higher-margin item to an existing lower-margin trip improves the basket's economics twice: once on the added revenue, and again on the mix.
For a brand-side analyst the same logic runs in reverse. If your SKU is the attachment rather than the anchor, your velocity depends on someone else's traffic and on merchandising you do not control. Knowing which role your item plays in the basket is the first question, and basket analysis is how you answer it.
The short version
- Attach rate is baskets containing both the anchor and the attachment, divided by baskets containing the anchor.
- It grows revenue from traffic already paid for, which makes it cheaper than any traffic-building alternative.
- Compute it per daypart, not weekly, and never confuse it with whole-store basket penetration.
- In convenience the attachable categories are also the highest-margin ones, so attachment improves the basket on revenue and on mix at the same time.