Key takeaways
  • Kanban is a signal that authorises replenishment. Nothing is made or moved without one, which caps work in progress by design.
  • The card count sets your inventory. Size it from demand, replenishment time and a deliberate buffer, then reduce it gradually.
  • The most common failure is producing without a card because the machine was free.
The rule that makes it work
No card, no production

Every kanban system that has degenerated into a push system did so by allowing exceptions to that one rule. A free machine is not a reason to make something; it is information about your bottleneck.

How the signal works

A downstream process consumes a container of parts and releases its card. That card travels upstream and authorises the production or movement of exactly one replacement container. Work in progress is therefore capped at the number of cards in circulation, whatever demand does.

Sizing the loop

  • Start from average demand during the replenishment lead time.
  • Add a buffer for demand variability and one for supply unreliability, and keep them separate so you know which you are paying for.
  • Divide by container quantity to get the card count.
  • Round up at first. Starting too lean creates stockouts that discredit the method.

Then reduce deliberately. Removing one card at a time and observing what breaks is how you find the real constraint, and it is the entire improvement mechanism.

Where small factories go wrong

  • Producing without a card to keep a machine busy. This is push wearing a kanban badge.
  • Cards that go missing, silently shrinking or growing the loop. Laminate them, count them monthly.
  • Using kanban on genuinely unstable demand or one-off products, where it is the wrong tool.
  • Never revisiting card counts after demand changes.

Where it fits and where it does not

Kanban suits repetitive, reasonably stable consumption: fasteners, packaging, sub-assemblies, consumables. It is a poor fit for bespoke work and for items with long, unreliable lead times, where a different planning approach serves better. Applying it universally is how it gets a bad reputation.