Kanban System Calculation
Kanban is a scheduling system for lean manufacturing that controls the flow of materials through a production system. The number of kanban cards needed depends on demand, lead time, container size, and desired safety stock.
Where:
- Demand = Average demand per time period
- Lead Time = Time to replenish inventory
- Safety Factor = Buffer for demand/supply variability (0-1)
- Container Size = Number of items per container
Results
Number of Kanban Cards Needed:
Total Inventory in System: units
Demand During Lead Time
Units needed during replenishment
Safety Stock
Buffer for variability
Containers Needed
Rounded up to full containers
Calculation Details
| Component | Calculation | Value |
|---|---|---|
| Demand During Lead Time | Demand × Lead Time | |
| Safety Stock | (Demand Variability + Lead Time Variability) × Safety Factor × Demand × Lead Time | |
| Total Required | Demand During Lead Time + Safety Stock | |
| Kanban Cards | Total Required / Container Size (rounded up) |
Kanban System Recommendations
- Start with the calculated number of cards and adjust based on actual performance
- Regularly review and reduce kanban cards to minimize inventory
- Consider implementing a two-card system (production and withdrawal kanbans)
- Use visual signals when physical cards aren't practical
Practical Examples
Example 1: Stable Demand
Demand: 100 units/day | Lead Time: 2 days | Container Size: 20 | Safety: 0.1
Kanban Cards: 11 (220 units total inventory)
Example 2: Variable Demand
Demand: 150 units/day (20% var) | Lead Time: 3 days (15% var) | Container Size: 30 | Safety: 0.2
Kanban Cards: 21 (630 units total inventory)
Example 3: High Variability
Demand: 80 units/day (30% var) | Lead Time: 5 days (25% var) | Container Size: 10 | Safety: 0.3
Kanban Cards: 38 (380 units total inventory)
Kanban System Guidelines
| Kanban Cards | Inventory Level | Recommendation |
|---|---|---|
| Too many | Excess inventory | Gradually reduce cards to expose problems |
| Optimal | Balanced flow | Monitor and fine-tune as processes improve |
| Too few | Stockouts | Add cards temporarily while fixing root causes |
Key Principles of Kanban
- Visualize the workflow - Make work and process visible
- Limit work in progress - Use kanban cards to constrain inventory
- Manage flow - Optimize the movement of work items
- Make policies explicit - Clearly define rules for the system
- Improve collaboratively - Continuously improve the system