How many kanban cards do you need? A formula and example
Size a kanban loop in a spreadsheet: lead-time demand, a safety factor and container size give the number of cards, shown with three worked parts.
The number of kanban cards in a loop is the demand during replenishment, plus a buffer, divided by the quantity one container holds and rounded up. Each card authorizes one container, so too few cards starve the line and too many hide problems.
How do you calculate lead-time demand?
Convert daily demand to an hourly rate using the hours the line runs, then multiply by the replenishment lead time. A fictional bracket needs 480 units a day over a 16-hour day, with a 4-hour lead time:
=B11/C11*D11 gives 480/16*4 = 120 units.
How do you add the safety factor and round?
A 25 percent safety factor lifts demand to 120*1.25 = 150 units. With 30 units per container:
=ROUNDUP(H11/F11,0) gives 150/30 = 5 cards.
Always round up. Four cards would cover only 120 units and leave no buffer.
What do the other parts need?
| Part | Daily demand | Lead time | Safety | Container | Cards |
|---|---|---|---|---|---|
| Bracket P-10 | 480 | 4 h | 25% | 30 | 5 |
| Housing P-20 | 320 | 6 h | 20% | 40 | 4 |
| Fastener kit P-30 | 800 | 8 h | 15% | 100 | 5 |
Housing: 320/16*6 = 120, times 1.2 is 144, divided by 40 is 3.6, rounded up to 4 cards. Fastener kit: 800/16*8 = 400, times 1.15 is 460, divided by 100 is 4.6, rounded up to 5 cards. The housing loop needs 4 cards; rounding up is where hand calculations slip. The kanban calculator does it per row.
When does this formula break down?
It assumes steady demand and lead time measured in the same operating hours as demand. Lumpy demand, long transport batches and changeover limits need a larger buffer or a different method. For reorder-point logic with variability, see the EOQ and reorder point calculator. To set the pace the loop must support, start from how to calculate takt time.