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Kanban Calculator: Number of Kanban Cards
How many kanban cards a pull loop needs.
15 cards of 50 units hold at most 750 units, 1.9 days of demand.
The pull loop, supplier to line
- SupplierLead timedaysemptied to back full
- Supermarketunits a card15 cards, 750 units, 1.9 days cover
- Lineunits a daysends back a card per container
Do this Take 0.5 day off the 1.5 day lead time: 5 fewer cards and 250 fewer units in the loop, 15 to 10 cards.
The kanban formula
Demand in units per day, lead time in days for the full replenishment loop, safety as a decimal (20% = 0.2), container size in units. Round up.
How to use this calculator
- Enter average daily demand for the part at the point of use.
- Enter the replenishment lead time in days, for the whole loop: collect the card, make or pick, move back.
- Enter the safety factor and container quantity in percent and units.
Worked example: a kanban loop for one part
An assembly cell uses 400 brackets a day. When a container empties, the card goes to the press shop, waits for the next pickup, is run and comes back: 1.5 working days in all. The team adds a 20% safety factor for demand and lead time variation. Brackets travel in containers of 50.
With safety = 600 × (1 + 0.20) = 720 units
Kanbans = 720 ÷ 50 = 14.4, rounded up to 15 kanbans
Most inventory in the loop = 15 × 50 = 750 units = 750 ÷ 400 = 1.9 days of demand
If the press shop cuts its loop to 1 day, the same sum gives 400 × 1 × 1.2 ÷ 50 = 9.6, so 10 cards and 250 fewer brackets in the loop. Shorter replenishment is what takes inventory out, not fewer cards on their own. The 1.5 day loop is loaded in the calculator above.
How to size a kanban loop, step by step
Two-bin kanban
A two-bin system is a kanban loop with N = 2. Instead of solving for the number of containers, you solve for the size of each bin: each bin must hold at least the demand during the replenishment lead time plus safety, D × L × (1 + S). In the example above that is 720 brackets per bin, which is why two-bin suits small, cheap parts better than brackets in 50-piece containers.
Kanban and Little's Law
The units in a kanban loop are work in process with a cap. By Little's Law, WIP = throughput × lead time, so the 750 units in the example at 400 a day equal about 1.9 days of lead time through the loop. Check other loops with the Little's Law calculator.
Do
- Measure the whole replenishment loop
- Round up, then remove cards one at a time
- Recalculate when demand or lead time changes
- Keep one card per container, always
Common mistakes
- Using only the machine run time as lead time
- Sizing to a peak day instead of leveling
- A large safety factor to cover an unreliable supplier
- Extra containers moving without cards
For the full method, card design and supermarket replenishment, read kanban in manufacturing. The glossary defines kanban, supermarket and pull system. To move a whole area to pull with your own numbers, see the Flow Starter Kit.
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Frequently asked questions
What is the kanban formula?
Number of kanbans N = D x L x (1 + S) / C, where D is average demand per day, L is the replenishment lead time in days, S is the safety factor as a decimal, and C is the quantity in one container. Round N up to a whole card. It is the same formula Toyota writes as N = D(Tw + Tp)(1 + a) / C, with waiting time plus processing time as the lead time.
How many kanban cards do I need?
Multiply average daily demand by the replenishment lead time in days, add the safety factor, and divide by the container quantity. With 400 units a day, a 1.5 day lead time, 20% safety and 50 units per container: 400 x 1.5 x 1.2 / 50 = 14.4, so 15 cards.
What lead time goes into the kanban calculation?
The whole replenishment loop, from the moment a container is emptied until it comes back full: time for the card to be collected, queue time at the supplying process, setup and run time, inspection, and transport back to the point of use. Using only the run time is the most common reason a kanban loop runs short.
Should I round the number of kanbans up or down?
Round up. Rounding down removes part of the safety factor and can starve the point of use. Once the loop runs reliably, take cards out one at a time and watch for shortages; each card removed lowers inventory by one container.
What safety factor should I use for kanban?
Start with a factor that covers the variation you have measured in daily demand and in the replenishment lead time, then reduce it as the loop proves stable. There is no universal value. If demand is very uneven, level it first; a large safety factor on an unlevel schedule only hides the problem.
What is a two-bin kanban system?
A loop with exactly two containers. When the first bin empties, it goes back as the signal to refill while the second bin is used. Each bin must hold at least the demand over the replenishment lead time plus safety: D x L x (1 + S). Two-bin works well for small, cheap parts such as fasteners.
What is the difference between kanban and safety stock?
Kanban sets how much inventory circulates in a pull loop to cover demand during replenishment; the safety factor inside it is a small allowance for variation. Safety stock is a buffer sized statistically from demand and lead time variation for a reorder point system. The safety stock calculator sizes that buffer.
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