CONWIP: a simple method to improve planning

By Pascal Pollet – Sirris, Belgium

In a previous article, we discussed the many advantages of limiting Work In Progress (WIP). Reducing WIP helps shorten and stabilise lead times in production. Stable lead times make production more predictable and planning simpler. In short, they increase overall efficiency. Less Work In Progress is achieved first of all by releasing fewer orders.

In this article we present two more advanced, card-based methods for limiting WIP: Kanban and CONWIP.

Kanban: one card per item in production

CONWIP stands for CONstant Work In Progress and is inspired by the Japanese Kanban system. Kanban comes from the Japanese ‘kan’ (visual) and ‘ban’ (card or board). The system was developed by Taiichi Ohno at Toyota to increase efficiency.

The Kanban system controls the material flow by attaching cards to the materials. These cards contain at a minimum the necessary information about the material:

• Item number
• Quantity
• Internal or external supplier
• Consumption site
• etc.

The accompanying card is detached as soon as a material unit starts being processed. It is then returned to the supplier or the previous work centre. The arrival of a Kanban card signals the need to produce or deliver a new quantity of the corresponding item.

A major drawback of the Kanban system is its limited suitability for production environments with a large variety of products. As product diversity increases, the number of Kanban cards rises proportionally, making the system more complex and less efficient to manage.

CONWIP: one card per amount of work

In a CONWIP (Constant Work in Progress) system, there is a fixed limit on the amount of work allowed in the entire process at any given time. This limit is controlled through CONWIP cards. Unlike Kanban cards, CONWIP cards are not product-specific — they represent a unit of work, such as a number of items, a pallet, a container, or a defined number of labour hours. As a result, the number of cards does not increase with product variety, making the system easier to manage in mixed-product environments.

When a product reaches the end of the production process, its accompanying CONWIP card is released and returned to the first process step. Only then can a new order be launched. The card itself does not determine which product will be produced; this is decided by consulting the production plan. The newly available CONWIP card is simply assigned to the next order in the queue.

In essence, CONWIP operates as a pull system. Work orders are initiated only when there is both demand and available WIP capacity. Production is therefore pulled based on available capacity, rather than pushed by the schedule. This ensures a steady, manageable workflow while reducing waste, overproduction, waiting times, and excessive inventory.

The key difference between Kanban and CONWIP lies in the role of the cards. In a Kanban system, cards represent inventory of materials or products, whereas in a CONWIP system, cards represent available production capacity — an amount of work that can be released into the system. 

Figure 1: Production line with 4 stations with a CONWIP system and 8 blue CONWIP cards. Four units of work are at the third station, indicating that this station is the bottleneck.