CONWIP stands for Constant Work In Process.

It is a pull-control method that limits the total amount of work allowed inside a production flow.

A new job is released only when an existing job exits and returns an authorization signal to the beginning of the system.

Understand the core rule

The basic logic is:

  1. a fixed number of authorization cards exist;
  2. each active job requires a card;
  3. a card returns when the job leaves the controlled flow;
  4. the returned card authorizes another job to enter.

The number of cards establishes the maximum authorized work in process.

Compare CONWIP with Kanban

Kanban typically controls replenishment between defined process or inventory points.

CONWIP controls the total WIP across a larger flow.

This can make CONWIP useful when:

  • product mix is high;
  • routings vary;
  • individual part-specific Kanban loops would be complicated.

The two systems can also be combined.

Use pull logic

Pull System releases work based on downstream consumption or available system capacity rather than pushing unlimited jobs into the process.

CONWIP applies that principle at the system level.

The release rule prevents excess WIP from accumulating simply because upstream resources are available.

Set the WIP limit deliberately

Too many CONWIP cards can create:

  • long lead time;
  • queues;
  • hidden problems.

Too few can cause:

  • starvation;
  • low throughput;
  • unstable utilization.

Little’s Law connects WIP, throughput, and lead time and helps teams understand the consequences of changing the WIP level.

The limit should be tested against real system performance.

Protect the constraint

Theory of Constraints emphasizes protecting the resource that limits system throughput.

A CONWIP release rule should avoid starving the constraint while still controlling total WIP.

The correct WIP level depends on variability, routing, and capacity conditions.

Define job sequence separately

CONWIP controls how much work enters the system.

It does not automatically define which job should enter next.

The organization still needs a dispatch or priority rule.

Examples include:

  • due date;
  • customer priority;
  • model sequence;
  • material availability.

Make authorization visible

The release signal may be:

  • physical card;
  • electronic token;
  • system status.

The rule should be difficult to bypass casually.

If planners continually release work without an available authorization, the WIP limit no longer exists.

Monitor system results

Useful measures include:

  • total WIP;
  • throughput;
  • lead time;
  • constraint starvation;
  • schedule attainment.

Flow Efficiency can help show how much elapsed time is truly value-creating versus waiting in the controlled flow.

Common mistakes

Treating CONWIP as only an inventory target, setting the card count without observing system behavior, releasing work without authorization, assuming CONWIP automatically determines job priority, starving the constraint, and reducing WIP too aggressively without understanding variability are common mistakes.

Practical sequence

  1. define the controlled production flow.
  2. measure current WIP and throughput.
  3. identify the constraint.
  4. define the initial CONWIP limit.
  5. create visible authorization signals.
  6. define the job-release priority rule.
  7. prevent unauthorized release.
  8. monitor throughput and lead time.
  9. adjust the WIP limit carefully.
  10. improve process stability so less WIP is needed.

The practical lesson

CONWIP controls the amount of work allowed into the system.

The goal is not minimum WIP at any cost; it is enough controlled WIP to sustain flow without hiding problems in queues.

This topic also connects with Work in Process (WIP). Use that method when the improvement requires the related operating or management discipline.