What is kanban?
Kanban (Japanese for "signboard" or "card") is a pull system: a process makes or moves parts only when a downstream process has used them and sent a card back. Each card represents one full container, so the number of cards fixes the maximum inventory in the loop. Developed by Taiichi Ohno at Toyota, kanban replaces forecast-driven scheduling at the point of use with a simple visual signal, preventing overproduction, one of the eight wastes.
The kanban formula
Number of kanbans = (D × L × (1 + S)) ÷ C, rounded up, where:
- D = average demand per day
- L = replenishment lead time in days (from empty container to full container back at the point of use)
- S = safety factor (e.g. 0.2 for 20%) to cover variation
- C = quantity per container (one card per container)
Maximum inventory = number of kanbans × C. Days of cover = maximum inventory ÷ D.
Kanban calculation example
An assembly cell uses 1,200 O-rings a day, delivered by the supplier two days after a card is released, in bags of 500. With a 20% safety factor: 1,200 × 2 × 1.2 ÷ 500 = 5.76, so the loop needs 6 kanbans and holds at most 3,000 O-rings, 2.5 days of cover. Machined housings come from an internal cell with a one-day lead time in boxes of 40: 400 × 1 × 1.25 ÷ 40 = 12.5, so 13 production kanbans. The tool prints each card numbered 1 of 13, 2 of 13 and so on, so a missing card is noticed.
The six rules of kanban
- The downstream process withdraws only what it needs, when it needs it.
- The upstream process produces only what was withdrawn, in the order it was withdrawn.
- No parts move or get made without a kanban.
- A kanban always travels with the parts.
- Defective parts are never sent downstream.
- Reducing the number of kanbans step by step drives improvement.
Kanban works best with level demand. Smooth the production mix with a heijunka box and cut batch sizes with SMED.
How to use this Kanban Calculator tool
- List the parts. Add each part in the loop with its part number, description and kanban type.
- Enter the loop data. Enter average daily demand, the full replenishment lead time in days, a safety factor and the container quantity.
- Read the result. The tool calculates the number of kanbans, the maximum inventory and the days of cover for every part.
- Add the locations. Enter where the parts come from and where they go, so they print on the cards.
- Print the cards. Select a part in the table to show its cards (one per container, numbered "1 of N"), then export to PDF or PNG and print.
- Review regularly. Recalculate when demand or lead time changes, and remove cards as the process becomes more reliable.
Frequently asked questions
How do you calculate the number of kanbans?
Number of kanbans = (average daily demand × replenishment lead time in days × (1 + safety factor)) ÷ container quantity, rounded up. For 1,200 parts a day, a 2-day lead time, a 20% safety factor and 500 parts per container: 1,200 × 2 × 1.2 ÷ 500 = 5.76, so 6 kanbans.
What is the difference between a production kanban and a withdrawal kanban?
A withdrawal (move or conveyance) kanban authorizes moving parts from a supplier or supermarket to the point of use. A production kanban authorizes a process to make a quantity of parts to replace what was taken. Many systems use both: withdrawal kanbans pull from the supermarket, production kanbans trigger refilling it.
What lead time should I use?
Use the full replenishment lead time: from the moment a container is emptied and its card released, through waiting, order processing, production or supplier lead time and transport, until the full container is back at the point of use. Underestimating it is the most common cause of stock-outs.
How big should the safety factor be?
The safety factor covers variation in demand and lead time. 10% to 30% is typical for stable demand and reliable supply. Start conservatively, then reduce the number of cards step by step as the loop proves stable: removing cards exposes problems to solve.
What information goes on a kanban card?
At minimum: part number and description, quantity per container, where it comes from (supplier or process) and where it goes (location), and the card number out of the total in the loop (e.g. 3 of 6). Many cards also show lead time, a barcode and the container type.
What is the maximum inventory in a kanban loop?
Maximum inventory is the number of kanbans × the container quantity: the stock in the loop if every container were full at once. Days of cover is that maximum divided by daily demand. Kanban caps inventory at this level, unlike forecast-based push systems.