392. Heijunka
Heijunka (平準化, “leveling”): a Toyota Production System technique for smoothing production by mixing products in a level cadence rather than running batches.
Heijunka makes takt time sustainable by absorbing demand fluctuations inside the production system.
392.1. Volume leveling vs mix leveling
Volume leveling: produce roughly constant total output each period (week, day, shift). Inventory absorbs demand variability over time.
Mix leveling: produce a mix of products each period, in repeating sequence, rather than running large batches of one product at a time.
Example: producing A, B, C in ratio 4:2:1 per day.
| Without heijunka | With heijunka |
|---|---|
| AAAA AAAA BB BB C | A B A A C A B A |
| Long batches; A inventory grows then drains | Steady draw on all parts; minimal inventory swings |
392.2. EPEI (Every Part Every Interval)
A key heijunka metric. EPEI = how often the line produces each product.
If products are scheduled to run, total setup time is , and available time per interval is :
Smaller EPEI = more frequent mix changes = more flexibility, but more setups.
Reducing setup times (SMED — Single-Minute Exchange of Dies) is the lever for reducing EPEI.
392.3. Heijunka box (load-leveling box)
A physical (or virtual) board that schedules production:
Time slot ──→
Product
A [kanban][kanban][kanban][kanban]
B [kanban]──────[kanban]──────
C [kanban]──────────────────Pull cards from left to right in each slot — produce the indicated product. Maintains the mix across time.
392.4. Benefits
- Smooth upstream demand — suppliers see steady draws on each part
- Low inventory swings — no boom-bust pattern from batch runs
- Customer-aligned — production cadence tracks demand
- Enables JIT — pull system needs level loading to function
392.5. Costs
- More setups — each mix change is a changeover (SMED becomes critical)
- Less efficient per setup vs long batches
- Worker training — operators must adjust between products frequently
- Tooling cost — quick-change fixtures, modular setups
392.6. When heijunka is harder
- High product variety with infrequent demand (long-tail SKUs)
- Long setup times not yet reduced via SMED
- Custom / engineered-to-order — no repeating sequence possible
- Volatile demand mix — sequencing must adapt frequently
In those cases, partial heijunka (level the high-volume products, run rest in batches) is a pragmatic compromise.
392.7. Implementation steps
- Calculate average demand per product per period
- Reduce setup times via SMED
- Establish a pitch (small repeating time slot)
- Sequence products in the heijunka box per the demand ratio
- Pull production based on the box; replenish customer pulls