🗂️ Kanban and Pull Systems Guide: Cards, Supermarkets, and the Number-of-Kanban Formula

What Is a Kanban?

A kanban is a signal that authorizes the production or movement of a defined quantity of material. The word is Japanese for "signboard" or "card," and the concept is central to the Toyota Production System and just-in-time (JIT) manufacturing. The rule is strict: nothing is produced and nothing is moved without a kanban. That single rule turns a chain of processes into a pull system, where actual downstream consumption — not a forecast — drives upstream activity.

Push vs. Pull

In a push system, a central schedule decides in advance what each process should make and when, based on a demand forecast. Each process produces to its schedule and pushes the output to the next process whether or not it is needed yet. Forecasts are never perfect, so push systems accumulate inventory where reality diverges from plan, and problems can be buried under that inventory for a long time.

In a pull system, a process produces only when a downstream process signals that it has consumed material. Inventory is capped by the number of kanban in circulation, so work-in-process (WIP) cannot grow without bound. Because buffers are small, problems surface quickly and demand attention — a feature, not a bug, of pull.

Attribute	Push	Pull (Kanban)
Trigger	Forecast / schedule	Actual downstream consumption
Inventory	Tends to accumulate	Capped by number of kanban
Problem visibility	Hidden by inventory	Exposed quickly
Best for	Stable, predictable demand	Repetitive demand with variety

How Kanban Cards Flow

The classic system uses two card types. A withdrawal (move) kanban authorizes moving a container of parts from a supermarket to the consuming process. A production kanban authorizes the upstream process to make a container of parts to replace what was withdrawn. As parts are consumed, cards detach and circulate back upstream, continuously signaling replenishment. The cards physically limit how much inventory can exist — there is exactly one container per card.

Supermarkets and Two-Bin Systems

A supermarket is a controlled, deliberately sized store of parts at the output of an upstream process. The downstream process withdraws from the supermarket like a shopper takes goods off a shelf; the gap left behind triggers the kanban that tells upstream to replenish exactly that amount. Supermarkets are used where continuous one-piece flow is impractical — for example, when an upstream machine serves many downstream lines or has long changeovers.

The simplest physical pull system is the two-bin system. Two bins of a part sit at the point of use. When the first bin empties, it becomes the replenishment signal (the empty bin or its card is the kanban) while the second bin covers demand during lead time. When the refill arrives, the cycle repeats. It is cheap, self-managing, and common for fasteners and consumables.

The Number-of-Kanban Formula

The number of kanban in a loop determines the maximum inventory and must be large enough to cover demand during the replenishment lead time plus a buffer. A widely used formula is:

K = (D × LT × (1 + α)) / C

• K = number of kanban cards (rounded up)
• D = average demand per unit time (e.g., parts per day)
• LT = replenishment lead time, in the same time units as D
• α = safety factor (a buffer, e.g., 0.10 for 10%)
• C = container (lot) size — parts per kanban

Worked example. Suppose demand is D = 800 parts/day, replenishment lead time is LT = 0.5 day, the safety factor is α = 0.20, and each container holds C = 50 parts. Then K = (800 × 0.5 × 1.20) / 50 = 480 / 50 = 9.6, which rounds up to 10 kanban. The loop will hold at most 10 × 50 = 500 parts.

The formula makes the levers explicit. Cutting lead time, shrinking container size, or reducing the safety factor all reduce the number of cards and therefore the inventory. This is why lean teams attack lead time relentlessly — it directly shrinks WIP. Note that demand during lead time (D × LT) is the same quantity that Little's Law relates to throughput and flow time, so the kanban loop is just Little's Law expressed as a card count.

E-Kanban

Physical cards can be lost, and they are awkward across long distances or many suppliers. E-kanban replaces cards with electronic signals — barcode or RFID scans at the point of use, or software triggers tied to an ERP/MES. When a bin is scanned empty, the system automatically issues a replenishment order to the supplying process or vendor. E-kanban preserves pull logic while adding real-time visibility, automatic reordering, and reliable record-keeping. Many organizations run hybrid systems: physical cards on the shop floor, electronic signals to external suppliers.

Rules That Make Kanban Work

• The downstream process withdraws only what it needs, only when it needs it.
• The upstream process produces only what was withdrawn, in the sequence withdrawn.
• No item is made or moved without a kanban.
• Defective parts are never passed forward — quality is built in at the source.
• The number of kanban is reduced over time to expose problems and drive improvement.

That last rule captures the philosophy: kanban is not just an inventory control method but a continuous-improvement engine. Deliberately removing a card stresses the system and reveals the next constraint to fix.