SC QUALITY GUILDKanban sizing calculator
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Enter each part's daily demand, replenishment lead time, safety factor and container quantity. Get the number of kanbans, the most stock the loop can hold and the days of cover it gives.
ON THE EXAM · CQE V.D.3, CQPA II.C.1, CSSBB VII.B.1, CSQP IV.A.3, CMDA V.B.3 · FREE · NOTHING YOU TYPE LEAVES YOUR BROWSER
Q-BotClick for an explanation of this tool.For educational use. This page is a study aid. The method behind it has far more depth and detail than one page and one tool can cover, and what you see here is not everything the method offers or everything this site offers. For full understanding and application, use the governing standards and texts, and seek further education, training or qualified advice. Results depend on the data you enter, so check them before relying on them.
What a kanban does
A kanban is a signal to make or move more. In a pull system nothing is made until a customer, or the next process, uses something. When a container is emptied at the point of use, its card (or the empty container itself) goes back to the supplying process as the order to refill it. The number of cards in the loop caps the stock: when every card is attached to a full container, the supplying process stops.
That cap is why sizing matters. Too few kanbans and the user runs out while the next container is still being made. Too many and the loop holds stock that hides problems, which is what pull is meant to stop.
The formula
The usual formula is
N = D × L × (1 + α) ÷ C, rounded up to a whole number,
- D is the average demand per day;
- L is the replenishment lead time in days: the card waiting to be collected, the queue at the supplying process, the changeover and run, and the move back;
- α is the safety factor, a percentage that covers variation in demand and lead time;
- C is the number of parts in one container.
D × L is the demand used up while one container is being refilled. The safety factor adds a margin, and dividing by C turns parts into containers. The result is rounded up, because a fraction of a card does not exist and rounding down guarantees a shortage. Some texts write the same idea as N = (D × L + safety stock) ÷ C.
Reading the result
- Max units in loop = N × C. It is the stock if every container were full. In practice some are empty and on their way back, so the average on hand is lower.
- Days of cover = N × C ÷ D. It is how long the loop could feed the user if the supply stopped.
- The chart splits each part's cover into lead-time demand, safety and rounding. A large gray block means the container is too big for the demand; a large gold block means the loop is carrying a lot of protection, usually for an unreliable process.
Making the loop smaller
Every term in the formula is a lever. A shorter lead time, from quicker changeovers (SMED), smaller batches or more frequent collection, cuts N directly. A steadier supplying process lets the safety factor come down. Smaller containers size the loop more closely. Lean practice is to set the loop to what the process can do today, then remove cards one at a time as the process improves, and watch what problem appears.
Recalculate when demand or lead time changes. A loop sized for last year's volume either starves the line or quietly builds stock. A leveled schedule, as in heijunka, keeps the daily demand on each loop steady.
On the exam
Kanban is one of the lean tools in the CQE Body of Knowledge (V.D.3). The CQPA lists pull, just-in-time and kanban among the lean tools (II.C.1), and the CSSBB includes pull systems and kanban under waste elimination (VII.B.1). The CSQP (IV.A.3) and CMDA (V.B.3) name kanban among the lean tools as well. Expect to calculate the number of kanbans from demand, lead time, safety factor and container size, to round up, and to explain what the number of cards controls.