Kanban systems are the best-known variant of pull systems and a cornerstone of lean manufacturing. Kanban systems help to maintain a good material availability in relation to the inventory. One of the main adjustments is the number of kanban cards in a kanban system. The number of kanban is a tradeoff between the cost of inventory, and the cost of a stock-out. Since kanban are used for make-to-stock production, a high availability of completed products is desired, often measured as the delivery performance (on time in full). At the same time excessive inventory is to be avoided to reduce inventory cost and to improve throughput. This paper describes a practical approach to estimate the impact of a reduction of kanban on the delivery performance, based on an analysis of the supermarket inventory.