Supermarket

Supermarkets are the central holding mechanism behind most kanban pull systems and one of the simpler ideas in lean once you see one in operation. The name comes from grocery stores, which Taiichi Ohno studied in the 1950s. Customers come in, take what they want, and grocers replenish the shelves overnight. The grocer does not produce milk and bread in response to forecasts; they refill what got taken. Bring that logic into a factory and you have a supermarket.

"Customer takes. Supplier replenishes. The shelf tells you what to produce next."

How a supermarket works

The physical setup is a defined location, usually labeled shelves or marked floor zones, with bins for each part variant. Each bin holds a target quantity. Each bin has a kanban card or other signal attached, sized to the bin's contents. Parts come into the supermarket from the supplier process and out of the supermarket to the customer process.

The mechanism runs on consumption. When the customer process takes a bin, the kanban card returns to the supplier process. The card is the signal to produce another bin's worth of that variant. The supplier produces, returns the refilled bin to the supermarket, and the card travels back attached to the new bin. The cycle repeats. The customer never has to plan or schedule; they just take what they need. The supplier never has to forecast; they just make what just got taken.

Sizing the supermarket is the thing most shops get wrong. Each bin's target quantity is calculated from three inputs: average consumption rate between refills, replenishment lead time from the supplier, and a sized cushion for variation. A bin that is too small stocks out and starves the customer process. A bin that is too large grows the supermarket beyond what flow requires, which is just relocated excess inventory. The math is straightforward but it requires honest data on consumption and supplier responsiveness.

Supermarkets are best used for runners and repeaters in runner-repeater-stranger classification. Steady or semi-steady demand justifies the stocked inventory. Strangers (one-offs, custom builds, sporadic orders) should not live in supermarkets; they bloat the floor with parts that may never sell again. Strangers run through sequenced pull or build-to-order instead.

Where a supermarket fits on the shop floor

Picture a small contract manufacturer building plastic injection-molded parts for a kitchenware brand. The brand orders four SKUs in steady volume, with weekly demand of about 500 units per SKU. Without a supermarket, the shop ran each SKU as a long batch every two weeks, which meant it sometimes ran out of one SKU while another was in production.

The shop installs a finished-goods supermarket near shipping. Each of the four SKUs gets a labeled shelf with three bins of about 200 units each. Each bin has a kanban card. When shipping takes a bin to fulfill an order, the empty bin and card go back to the molding line. The molders produce a 200-unit batch of that SKU, fill the bin, and return it to the supermarket. The schedule for the line is now whatever cards have come back; sales no longer pushes long-batch decisions onto the molding floor.

Within a quarter, stockouts stop. Lead time to ship orders drops from four days to one. Total inventory across the four SKUs is lower than it was under long-batch scheduling, because the supermarket is sized rather than padded. The molding line runs on whatever the cards say, in small frequent batches, and the cards faithfully report what shipping just consumed.

Common mistakes with supermarkets

• Sizing bins by gut feel. Each bin should be sized from consumption rate times refill lead time, plus a defined cushion. Anything else is a stockroom.
• Stocking strangers. Supermarkets are for runners and repeaters. One-offs do not belong here; they create dead stock.
• No bin labels or cards. Without clear labels and signals, the supermarket becomes a hunt and pull breaks down.
• Letting it grow uncapped. A supermarket without target sizes per bin drifts upward over time. The sizing is what keeps it from becoming a warehouse.
• Refilling from forecasts instead of consumption. The whole point is that the supplier refills what got taken. Refilling from a separate schedule defeats the mechanism.

Supermarket and related Lean tools

A supermarket is the central store inside any kanban loop, the closed circuit a signal travels between supplier and customer processes. It is the most common implementation of pull system mechanics, sized to runners and repeaters identified through runner-repeater-stranger classification. The simplest possible supermarket is a two-bin system, with two bins acting as both the store and the signal mechanism.