Heijunka, or production leveling, spreads both the volume and the mix of work evenly across time. Instead of long runs of one product followed by long runs of another, the schedule makes a small, repeating mix of everything the customer buys, so the line and its suppliers see a steady rhythm.
It is the discipline that lets a pull system work. By removing unevenness (mura) at the point where the schedule is set, heijunka reduces the extra inventory, capacity and overtime that erratic demand would otherwise force on every upstream process.
Why Heijunka Matters
Stabilizes the Whole Flow
Level output means steady demand on every upstream process, supplier and worker, instead of surges followed by idle time.
Cuts Inventory and Lead Time
Building small amounts of every product often means finished goods and WIP stay small while customers still get what they want.
Makes Kanban Workable
Pull systems assume reasonably steady demand. Leveling gives them that, so cards do not swing wildly.
Reveals the Real Constraints
Once you try to produce a mix every hour, changeover time, quality problems and material shortages have nowhere to hide.
What Heijunka Is
Heijunka (level production, or production smoothing) is the Toyota Production System practice of spreading both the volume and the mix of work evenly over time. Rather than building all of Product A on Monday and all of Product B on Tuesday because that is convenient for the machines, a heijunka schedule makes a small, repeating mix of every product each shift, each hour or each pitch.
It attacks mura, unevenness, which the Toyota system treats as a root cause of both muda (waste) and muri (overburden). Uneven demand forces plants to carry extra capacity and inventory to absorb the peaks. Level scheduling removes the need for both. See the 8 Wastes Guide for the wastes this reduces.
Core Terms
| Term | Meaning |
|---|---|
| Takt time | Available production time divided by customer demand; the pace at which a unit must be completed. |
| Pitch | Takt time multiplied by the pack-out quantity: the time interval in which the schedule is released and checked. |
| Heijunka box | A wall grid with a row per product and a column per pitch, holding the cards that release work in a leveled sequence. |
| Leveling by volume | Producing the same total quantity each period even when orders arrive unevenly, by drawing on a finished-goods buffer. |
| Leveling by mix | Alternating products in small, repeating quantities instead of long runs of one product. |
| EPEI | Every Part Every Interval: how often a product is built. A shorter interval means smaller batches and a smoother flow. |
| Load leveler | The person who maintains the box and releases the cards, often called the water spider or production controller. |
How to Level a Schedule
- Calculate takt time from real demand (see the Takt, Cycle and Lead Time Guide and the Takt Time Optimizer).
- Smooth the demand. Convert orders into a steady daily quantity by product, using a small finished-goods buffer to absorb ups and downs.
- Compute the mix. Express each product's share of demand as a ratio, such as 6 : 3 : 1.
- Choose the pitch. Multiply takt by a practical pack-out quantity so the interval is short enough to be useful and long enough to manage.
- Build the sequence so each pitch contains the same mix, and spread the largest-volume product evenly among the others.
- Load the heijunka box and release one pitch's cards at a time to the line, using the box to watch whether each pitch is met.
- Reduce changeover time (SMED) until the mix you want is affordable, then shorten the interval again.
Worked Example: Three Products, One Line
A cell makes three product variants. Customers order 100 units per day in a 6 : 3 : 1 mix, so there are 60 units of A, 30 of B and 10 of C. The line has 450 minutes of production time per day. The numbers are illustrative.
- Takt time = 450 min / 100 units = 4.5 minutes per unit.
- Packing 10 units at a time gives a pitch of 10 × 4.5 = 45 minutes, or 10 pitches per day.
- One leveled pitch contains 6 A, 3 B and 1 C. A unit-by-unit sequence that spreads them is: A B A A B A C A B A.
The trade-off is changeovers. Compare three ways to run the same day, assuming a 2-minute changeover between products (illustrative):
| Approach | Changeovers per day | Time spent changing over | How often the customer can get Product C |
|---|---|---|---|
| Batch by product (A, then B, then C) | 2 to 3 | About 4 to 6 min | Once per day |
| Level by 90-minute interval (12 A, 6 B, 2 C per interval) | 15 | 30 min | Every 90 minutes |
| Level by pitch (6 A, 3 B, 1 C per 45 minutes) | 30 | 60 min | Every 45 minutes |
| Level unit by unit (sequence above) | 80 | 160 min | Every 10 units |
The right answer depends on your changeover time and capacity headroom. With a 2-minute changeover, unit-by-unit leveling is not affordable: 160 minutes would consume more than a third of the day. The practical path is to level at the coarsest interval that your customers and inventory targets can accept, then use SMED to cut changeover time and step the interval down toward the pitch. This is exactly how Toyota-style plants progress: heijunka is the destination and quick changeover is the road.
Changeover counts assume the run order repeats: 3 blocks per 90-minute interval gives 3 changes per interval, 15 over the 450-minute day. All approaches also require enough spare capacity to absorb the changeover minutes.
How Heijunka Works With Kanban
A Kanban system reacts to consumption, so if the final assembly line asks for a huge run of one product and then nothing, the cards will surge and the upstream processes will follow. Level the pull at the pacemaker process, the one point in the value stream that is scheduled directly, and every upstream supermarket and supplier sees a smooth, predictable draw. See Kanban Pull Systems for how to size the loops, and the Kanban Quantity Calculator to work the numbers.
Self-Assessment Questions
- Do we know our real takt time and the mix of demand by product?
- Do upstream processes see steady demand, or big surges for each product?
- What is our changeover time between each pair of products, and is it improving?
- Do we have a small finished-goods buffer to absorb demand swings without disturbing the schedule?
- Is someone responsible for loading the box and checking each pitch was met?
Common Mistakes
Leveling Before Fixing Changeovers
Forcing a small-lot mix on a line with long changeovers just converts waiting time into downtime. Reduce changeover time first or in step.
Leveling the Schedule but Not the Demand
If wild order swings flow straight through, the line is still whipsawed. A buffer and a stable order-release rule are part of heijunka.
Ignoring the Mix Because Volume Looks Level
Steady total volume with erratic mix still overloads certain processes and materials on some days.
Treating the Box as Decoration
A heijunka box no one checks each pitch is just a board on the wall. The discipline of checking against plan is where problems surface.
Quick Reference
Formulas
- Takt = available time / demand.
- Pitch = takt × pack-out quantity.
- Mix ratio = each product / total demand.
- Changeovers per day = changes in the repeating sequence × repeats.
Steps to Get Started
- Choose one product family and one pacemaker process.
- Start with a coarse interval, such as a shift or half shift.
- Run SMED on the longest changeovers.
- Step the interval down as changeovers improve.
Heijunka (Production Leveling): Frequently Asked Questions
What is heijunka in lean manufacturing?
Heijunka is the Toyota Production System practice of leveling production by volume and by mix, so that a plant builds a small, repeating assortment of every product at a steady rate instead of building large batches of one product at a time. Its aim is to remove unevenness, which reduces inventory, overburden and waste across the flow.
What is a heijunka box?
A heijunka box is a wall-mounted grid with a row for each product and a column for each time interval, or pitch. Cards for the work to be built in each pitch are placed in the slots, and a load leveler releases one pitch's cards at a time to the line. The box makes the leveled plan visible and shows immediately when a pitch is missed.
How do you calculate pitch?
Pitch is takt time multiplied by the pack-out quantity, the number of units that move together in a container or pack. For example, with a takt time of 4.5 minutes and a pack-out quantity of 10 units, the pitch is 45 minutes. The schedule is released, and progress is checked, once per pitch.
Sources and Further Reading
- Art Smalley, Creating Level Pull, Lean Enterprise Institute.
- Mike Rother and John Shook, Learning to See.
- Taiichi Ohno, Toyota Production System: Beyond Large-Scale Production.
- Jeffrey K. Liker, The Toyota Way.