Kanban: cards, containers and how many are needed

In one line

Kanban is a visual signal (a card, a container or a square painted on the floor) that authorizes a workstation to produce or replenish material, and limits how much can pile up.

What it is

Meyers and Stephens define it as a simple, direct form of communication that is always placed at the point where it is needed. It is usually a small card inside a plastic sleeve, with information such as the part number, the quantity per container and the delivery point. The card tells the operator to produce the quantity that left the previous process, and it is used to manage and secure just-in-time production; instead of cards, containers or a kanban square can be used.

The Universidad Icesi paper places kanban within just in time: it controls the transformation and movement of materials with visual devices, so that the quantity and location of material in the process are controlled by design. It is the tool of a pull system.

How to calculate the number of cards

The Minitab Engage documentation gives the formula for the size of the "supermarket" (the replenishment stock between two workstations):

• Supermarket size (units) = demand rate × replenishment time × (1 + safety stock).
• Number of cards = supermarket size expressed in containers (that is, divided by the units in each container and rounded up).

The demand rate is the speed at which inventory is consumed; the replenishment time includes making, transporting and waiting for a container; the safety stock is the extra percentage that protects against stoppages. The same source describes variants: with a single container per unit, with two containers (one being consumed and one being replenished, for a total of twice the units per tray) and with several containers, where the "trigger point" indicates when replenishment is set off.

Calculate your cards

Illustrative example

A workstation consumes 100 parts per hour. Replenishing a container (making, moving and waiting) takes half an hour, and 10% safety stock is added. The material in circulation is 100 × 0.5 × 1.10 = 55 parts; with containers of 25 parts, 3 cards are needed. If the replenishment time drops to 15 minutes (for example, with SMED or by moving the workstations closer together), 2 are enough. This is an example with made-up numbers to show the mechanism: in your plant the values come from measuring.

Benefits

• Inventory between workstations is limited by design, not by good intentions.
• A clear visual signal: you do not need a computer system to know what is missing.
• When the replenishment time goes down, the number of cards goes down on its own: the calculation shows the benefit of each improvement.

Limitations to keep in mind

• It works well with repetitive, stable flows; it is not suitable for parts with very irregular demand or low consumption.
• It requires discipline: if cards are added "just in case," the inventory you wanted to get rid of comes back.
• The calculation assumes a stable replenishment time; if it is highly variable, the safety margin skyrockets.

In summary

Kanban turns the idea of "produce only what was consumed" into a physical rule: one card, one container. To size it, three inputs are enough (demand, replenishment time and safety margin), and the result is an exact list of how much inventory is accepted between two workstations.