Kanban Calculation: How Many Cards and Containers Do You Need?

Define what one card authorizes

In manufacturing, a kanban signals permission to produce or move a specified quantity. The Lean Enterprise Institute distinguishes production kanban from withdrawal kanban. A task board with columns does not answer how much physical inventory a machining cell should hold.

Write the part number, supplying process, consuming process and container quantity before calculating anything. A returned card must mean the same thing to the operator, material handler and planner. Digital signals need the same discipline as paper cards.

This guide sizes one replenishment loop. Use value stream mapping to locate it and Little’s Law to examine broader flow; neither replaces the card authorization rule.

Calculate coverage in units first

Let D be average demand in units per working hour, L the elapsed working hours from consumption signal to usable replenishment, s a declared fractional allowance, and C units per container. Include queueing, transport and release delays in L, not just machine cycle time.

The allowance in this simple model is a planning assumption. A 20% value has no automatic statistical service guarantee. If you already added safety units to the numerator, do not multiply by another safety factor without explaining why.

An 18-card loop, and what changes it

Illustrative example: one part is consumed at 200 units per working hour. Replenishment takes 1.5 working hours. Containers hold 20 units and the team initially allows 20% extra coverage. Demand and container sizes are held constant for this teaching case.

Coverage is 200 × 1.5 × 1.20 = 360 units. Dividing by 20 gives 18 cards. That is the authorized quantity across the defined loop, not 360 units at every location. Count containers in use, replenishing and waiting according to the loop design.

Check the route on the shop floor

Record signal time and the time a replacement container becomes usable at the consuming point. A receipt in the warehouse is too early if the material still needs inspection or internal transport.

Separate normal cycles from exceptional delays. Averages can under-cover a route with frequent long tails. High-mix lines also need a replenishment sequence: a card cannot create capacity for a product that will not run until Friday.

1. Observe several replenishments

   Capture demand, signal, queue, production, inspection and delivery times for the same part family.

2. Set a documented initial allowance

   Name the variability it covers and the service failure it is meant to avoid. Do not copy another line’s percentage.

3. Pilot a bounded card count

   Agree shortage reporting and recovery with the planner; keep safety and customer commitments under competent site control.

4. Review stock and misses together

   Track shortage frequency, recovery effort, full and empty containers, and unauthorized production before resizing.

Avoid a separate safety pile outside the loop

Unlabelled emergency stock makes the card count look lean while total inventory remains high. Inventory kept outside the loop should have an explicit purpose, owner and review date.

For intermittent demand, minimum supplier lots or long campaigns, a single average-rate formula may be a poor fit. Consider product-family supermarkets, order-driven replenishment or a separately designed buffer. The lean manufacturing guide explains pull principles; this calculation assumes the chosen part is suitable for replenishment.

Use digital kanban to improve the record

An electronic card can retain signal timestamps, failed acknowledgements and container identity. Those records help distinguish lost signals from genuinely inadequate coverage. Count one authorization once even if a message is retried.

A useful first improvement is often shorter and more predictable replenishment, followed by a controlled reduction in cards. State the shortage risk and review period before changing a live line.