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Method guide · Value stream mapping and flow

Kanban: meaning, how the system works, card types and the six rules

Short answer

Kanban is a visual signal, usually a card, that tells the previous process what to make or move and how many. The word is Japanese for signboard or sign card. In a kanban system nothing is produced until a card arrives, so material is pulled by actual use instead of pushed by a forecast.

A kanban system is a set of cards (or bins, or electronic signals) that travel in a loop between a using process and a supplying process. When the user empties a container, its card goes back as the order to refill it. The number of cards in the loop caps the inventory. The same idea, a card per piece of work and a limit on how many are in progress, is what a kanban board does for tasks in an office or software team.

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What does kanban mean?

Kanban is a Japanese word, usually translated as signboard or sign card. In lean manufacturing it means the card or signal that authorizes a process to make or move a fixed quantity of parts. The card says what the part is, how many go in a container, where it comes from and where it goes.

The card itself is not the point. The point is the rule behind it: no process makes anything, or moves anything, until a kanban tells it to. That rule turns a plant from push, where each step produces to a schedule and hopes the next step needs it, into pull, where each step produces only what the next step has actually used. Pull system in lean explains push against pull in full.

People use the word in three ways, which is why the meaning seems to shift between articles. Kanban can be the card. It can be the system of cards and rules that runs a pull loop. And since the mid 2000s it can be the kanban method for knowledge work, where a board with columns and work-in-progress limits replaces the cards and containers. This page covers all three, starting with the shop floor, where the method began.

Where kanban came from

Taiichi Ohno developed kanban at Toyota in the early 1950s as the tool that made just-in-time production workable. In his book Toyota Production System: Beyond Large-Scale Production (English edition, Productivity Press, 1988) he describes taking the idea from the American supermarket: the customer takes what is needed from the shelf, and the shelf is restocked according to what was taken. He applied that to the plant. The later process is the customer, the earlier process is the shelf, and the kanban is the shopping list.

That is why a store of parts between two processes in a lean plant is still called a supermarket, and why a kanban loop runs between a supermarket and the process that fills it. Supermarket pull systems covers how to set one up. Kanban sits inside the just-in-time pillar of the Toyota Production System, alongside takt time and continuous flow; just-in-time manufacturing places it in that bigger picture.

How a kanban system works

A kanban system is a loop. Follow one container of parts around it and the whole method is visible.

One kanban card going round the loop

  1. 1

    The using process takes a full container from the store

  2. 2

    The card comes off when the first part is used

  3. 3

    The card travels back to the supplying process

  4. 4

    The supplying process makes exactly what the card says

  5. 5

    The full container, card attached, goes back to the store

Back to step 1 when the container is used

Nothing is made until a card comes back, so the number of cards caps the stock in the loop.
  1. 1

    The using process takes a full container from the store

    An assembly cell runs out of brackets and takes the next full container from the supermarket. The kanban card is attached to that container.

  2. 2

    The card comes off when the first part is used

    The operator takes the card off the container and drops it in the kanban post. Many plants take it off at the first part, not the last, so the refill starts before the container is empty.

  3. 3

    The card travels back to the supplying process

    A material handler, often on a fixed route (the water spider), collects the cards from the post and takes them to the press that makes brackets. The card is now a production order.

  4. 4

    The supplying process makes exactly what the card says

    The press makes one container of brackets, the quantity printed on the card, in the order the cards arrived. With no card it makes nothing and switches to another part or stops.

  5. 5

    The full container, card attached, goes back to the store

    The container returns to its marked place in the supermarket with the card on it, ready for the cell to take again. The loop is closed.

Because every container in the loop carries a card, and the cards are counted, the inventory between the two processes cannot grow past the number of cards times the container quantity. That is the control. Overproduction, the waste that hides all the others, becomes impossible without breaking a rule that everyone can see.

A visual board often sits beside the loop: a heijunka box or a scheduling board where the returned cards are slotted in the order the supplying process should run them. Kanban pull systems and visual scheduling shows how the cards become the day's sequence.

Types of kanban card

Most plants use two kinds of card: one that orders production and one that orders movement. The rest are variations for particular situations.

Kanban card types and when each is used
TypeWhat it authorizesTypical use
Production kanbanMake one container of the part shownBetween a supermarket and the process that fills it
Withdrawal (move) kanbanMove one container from the store to the point of useBetween a supermarket and a using line some distance away
Signal (triangle) kanbanStart a batch when stock falls to the marked levelProcesses with long changeovers such as presses and moulding, where parts are made in batches
Supplier kanbanThe supplier delivers one container of the partExternal suppliers on a regular delivery route
Two-bin kanbanRefill the empty binSmall, cheap parts: fasteners, labels, consumables
Electronic kanban (e-kanban)The same signal sent as a message or scanLong distances, external suppliers, or where cards get lost

A two-bin system is a kanban with the bin as the card: when the first bin is empty it goes back for refilling and the line works from the second. [Kanban card types](/blogs/kanban-card-types-two-bin-electronic) goes through each type with examples of what the card carries.

Whatever the type, a card carries the same minimum: part number and description, quantity per container, the supplying location, the using location, and a card number out of the total (card 4 of 10), so a missing card is noticed. The free kanban card template has six production and six withdrawal cards to print, with a card register.

How many kanban cards do you need? A worked example

The number of cards sets the inventory in the loop, so it is the one calculation that matters. A common formula is: cards = daily demand x replenishment lead time in days x (1 + safety factor) / quantity per container, rounded up to a whole card. The lead time is the whole trip of a card around the loop, including the time it waits in the post and the queue at the supplying process, not only the run time.

This example is illustrative. An assembly cell uses 240 brackets a day. The press that makes them takes 1.5 days to turn a card round: half a day waiting in the post and on the route, half a day in the press queue, half a day to run and return. A container holds 40 brackets. The team adds a 10% safety factor for the days demand runs high.

Illustrative kanban sizing for one part

Example numbers

Cards = demand over the lead time × (1 + safety factor) ÷ container quantity, rounded up

Check it the other way: 10 cards x 40 = 400 brackets, and 400 / 240 a day = 1.67 days of cover against a 1.5 day lead time, which is the 10% margin. The free kanban sizing calculator does this per part and shows the change from today's card count.

  1. Demand over the lead time =240 brackets a day x 1.5 days= 360 brackets
  2. Add the safety factor =360 x (1 + 0.10)= 396 brackets
  3. Divide by the container quantity =396 / 40= 9.9 containers
  4. Round up =9.9 rounds up to the next whole card= 10 cards
  5. Maximum stock in the loop =10 cards x 40 brackets= 400 brackets, about 1.7 days of demand
Round up10 cards
Always round up: 9.9 containers is 10 cards. The extra fraction of a card is part of the safety margin.

Two things move that number. Shorten the lead time, by collecting cards more often or cutting the press queue, and the cards fall. Shrink the container and more cards are needed but each holds less, so the store takes up less space and the loop reacts faster. If the store never drops below two full containers, a card can come out. Once the loop is running, a plan for every part holds the demand, container and lead time for every part number so the sizing can be redone when any of them changes.

The six rules of kanban

A kanban system only works if everyone follows the same rules, because a single process that makes parts without a card puts stock into the loop that nothing accounts for. Ohno set out six rules in Toyota Production System. In plain words:

  • Take only what you need. The next process withdraws only the quantity on the kanban.
  • Make only what was taken. The supplying process makes the quantity the cards show, in the order they arrived.
  • No kanban, no work. Nothing is made or moved without a card.
  • The kanban stays with the parts. A card is always attached to the goods it describes.
  • Never pass on a defect. Bad parts do not go into a container with a card on it.
  • Reduce the number of kanban over time. Less stock in the loop, so problems show up sooner.

The sixth rule is the one that makes kanban an improvement tool rather than a stock-control tool. Each card removed lowers the water level, and the next problem that was hidden under inventory, a slow changeover, a machine that stops, a route that runs late, comes into view and gets fixed. The free kanban rules poster puts the six rules on the wall at the supermarket and the kanban post.

Kanban in lean manufacturing: where it fits

Kanban is the mechanism of pull, and pull is one of the three parts of just-in-time alongside takt time and continuous flow. Where two processes can be joined into one-piece flow, there is no need for kanban between them: the part passes hand to hand. Kanban is for the places where flow is not yet possible, because of a changeover, a shared machine, a distance, or a supplier. The future-state value stream map marks those points and puts a supermarket and a kanban loop at each.

Kanban compared with the alternatives at a process boundary
Link between two processesHow material movesUse it when
Continuous flowOne piece at a time, no storeCycle times are close and the processes can sit together
FIFO laneA marked lane with a maximum, first in first out, no cardsProcesses cannot be joined but make the same sequence of parts
Supermarket with kanbanA store of every part, refilled by cardsThe supplying process serves several customers or has long changeovers
Push to scheduleEach process runs its own scheduleNothing pulls; this is what kanban replaces

Kanban loops also need a delivery route to carry the cards and containers. The water spider, or mizusumashi, runs that route on a fixed timetable: see [mizusumashi, the water spider](/blogs/mizusumashi-water-spider-material-delivery).

The common failure is to install cards without the discipline around them. Cards go missing, operators make parts ahead to be safe, the store overflows, and the plant concludes that kanban does not work. It was never running. The rules above, a daily check of the card count, and a leader who asks why a part was made without a card are what keep a loop alive.

Kanban boards for tasks and projects

Software teams took the same idea in the mid 2000s and applied it to work that has no containers: a card per task, a column per stage (to do, in progress, review, done), and a limit on how many cards a column may hold. When a column is full the team finishes something before starting something new. That is the work-in-progress limit, and it does for a task board exactly what the card count does for a parts loop: it caps the work in process and shortens the time each item takes to get through.

The kanban method for knowledge work is usually summed up as four principles (start with what you do now, agree to pursue incremental change, respect current roles, encourage leadership at every level) and six practices (visualize, limit work in progress, manage flow, make policies explicit, use feedback loops, improve together). The four core principles and six practices of kanban goes through each.

On a shop floor the board version is useful for improvement actions, maintenance work orders and audit findings: work that otherwise lives in a spreadsheet nobody opens. The free kanban board is a four-column board with WIP limits and sample cards. Keep the two meanings apart when you talk to people: a manufacturing kanban board is usually the scheduling board where returned cards are sequenced, not a task board.

Common mistakes

  • Sizing the loop on run time alone. The lead time in the formula is the full trip of a card: waiting in the post, travel, queue at the supplier, run, and return. Leave out the waits and the line runs out.
  • Making parts without a card to be safe. One process doing this puts unplanned stock into the loop and hides the shortage that should have been fixed.
  • Never recounting the cards. Demand changes, lead times change, cards get lost. A loop sized once and never checked drifts either into shortages or into a store that overflows.
  • Starting kanban between every pair of processes. Join processes into flow where you can, use a FIFO lane where the sequence is fixed, and save kanban for the boundaries that need a store.
  • Treating the task-board version and the shop-floor version as the same thing. The idea is shared; the sizing, the rules and the cards are not.
  • Removing the sixth rule. If the card count never falls, the loop stops exposing problems and becomes a reorder-point system with extra paperwork.

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Work items in the stages of a process, with a work-in-progress limit per column.

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FAQ

Questions about kanban

  1. Kanban is a Japanese word usually translated as signboard or sign card. In lean manufacturing it is the card or signal that authorizes a process to make or move a fixed quantity of parts. The same word names the system of cards and rules that runs a pull loop, and the kanban method for managing tasks on a board.

  2. A kanban system is a loop of cards between a process that uses parts and a process that makes them. When a container of parts is taken, its card goes back as the order to make one more container. Nothing is made without a card, so the stock between the two processes can never grow past the number of cards.

  3. An assembly cell takes a container of 40 brackets from a store and puts the card in a post. A material handler carries the card to the press, which makes 40 brackets and returns them with the card to the store. With ten cards in the loop the store never holds more than 400 brackets. A two-bin rack for screws, where the empty bin is the signal to refill, is the simplest version.

  4. Taiichi Ohno's six rules, in plain words: take only what you need; make only what was taken, in the order the cards arrived; no kanban, no work; the kanban stays with the parts; never pass on a defect; and reduce the number of kanban over time so problems show up sooner.

  5. Cards = daily demand x replenishment lead time in days x (1 + safety factor) / quantity per container, rounded up. For 240 parts a day, a 1.5 day lead time, a 10% safety factor and 40 per container: 240 x 1.5 x 1.1 / 40 = 9.9, so 10 cards. The lead time is the full trip of a card round the loop, not only the run time.

  6. Production kanban orders a container to be made. Withdrawal or move kanban orders a container to be moved from the store to the point of use. Signal kanban starts a batch when stock falls to a marked level. Supplier kanban goes to an outside supplier. Two-bin uses the empty bin as the card, and electronic kanban sends the same signal as a scan or message.

  7. On the shop floor kanban is a card attached to parts that tells the previous process what to make. A kanban board, in the knowledge-work sense, is a board with a column per stage, a card per task and a limit on how many cards a column may hold. Both cap work in process; one does it with a card count, the other with a column limit.

  8. Both, in different forms. Kanban began as a lean manufacturing tool at Toyota in the 1950s, the mechanism that pulls material through a plant. In the mid 2000s software teams adapted it as the kanban method, which is counted among the agile approaches. The shared idea is to limit work in progress and let demand pull the work.

  9. Scrum works in fixed-length sprints with a planned batch of work, set roles and a review at the end of each sprint. Kanban has no sprints: work flows continuously through the board, limited by work-in-progress caps, and can be started or reprioritized at any time. Teams often combine the two.

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