Lean teams use heijunka to stop the peaks and troughs that create overtime, idle time and excess inventory. A steady schedule also makes pull systems and standard work stable.
Where it comes from
Heijunka is a foundation of the Toyota Production System, sitting under just-in-time and jidoka. A heijunka box is the common tool for releasing kanban in a leveled sequence.
At a glance
From weekly blocks to a leveled mix
Swipe sideways to see the whole diagram.
How to apply Heijunka
- 1
Understand demand
Analyze demand by product over several periods. Separate steady runners from low-volume or erratic items.
- 2
Calculate takt time
Divide available time by average demand. Takt sets the pace the leveled schedule must hit.
- 3
Shorten changeovers
Use SMED so the line can switch products often. Leveling mix is impractical while changeovers stay long.
- 4
Build a leveled pattern
Create a repeating sequence that mixes products in proportion to demand. Decide the pitch, or time interval, for releasing work.
- 5
Use a heijunka box
Load kanban into time slots by pitch so work is released at a steady rate. Material handlers pull one slot per interval.
- 6
Buffer and review
Hold a small finished-goods buffer to absorb short-term swings. Review the pattern when demand shifts.
Worked example (illustrative)
A line sells 200 of model A, 200 of B and 100 of C each week, and builds them in blocks: A on Monday and Tuesday, B on Wednesday and Thursday, C on Friday. Stock of A peaks near a full week of demand while C runs short by midweek. The team cuts changeover time and moves to a daily A B A B C pattern of 40 A, 40 B and 20 C. Finished stock of each model now stays near one day of demand, about a fifth of the old peak, and the downstream schedule becomes predictable.
Common mistakes
- Trying to level the mix before reducing changeover times.
- Chasing every short-term order change, which reintroduces the peaks heijunka should remove.
- Leveling final assembly while upstream processes still run in large batches.
When Heijunka is not the right tool
Heijunka needs some repeat demand to level. Highly custom work with no repeating products is hard to level by mix.
A common approach is to split products into runners, repeaters and strangers, then level the runners and repeaters first.
Heijunka box and board
A heijunka box is a board with a grid of slots. Columns are time intervals, set by the pitch, and rows are products. Kanban cards are placed in the slots to show what to make in each interval, in a mixed and level sequence.
A material handler collects the cards for each interval and takes them to the line. Because the box holds the plan, everyone can see whether production is on time just by looking at which slots are empty.
Pitch sets the width of each column: takt time multiplied by the pack quantity. In the illustrative example above, 100 units a day in 450 minutes gives a takt of 4.5 minutes. With 20 units per container, the pitch is 90 minutes, so the box has five slots a day for the A B A B C pattern.
Art Smalley's guide Creating Level Pull describes how leveling, pull and kanban work together.
Leveling volume and mix
Volume leveling makes the same total amount each day, even when orders arrive unevenly. Mix leveling spreads the different products across the day instead of in long batches.
Both need short changeovers, which is why SMED usually comes first.
Guides on this topic
- How to do value stream mapping, step by stepHow to do value stream mapping step by step: walk the process, record data, draw the current state by hand, design the future state, then add software.Read the guide
- Lean manufacturing examples from shop floor to officePractical examples of lean in action: what the problem looked like, which lean tool was used and what changed, in factories and beyond.Read the guide
Sources
Books and papers show their publication year. Web pages show the month we last checked them.
- 1.Art Smalley, Creating Level Pull (Lean Enterprise Institute, 2004)
- 2.Jeffrey K. Liker, The Toyota Way: 14 Management Principles from the World's Greatest Manufacturer (McGraw-Hill, 2004)
- 3.Yasuhiro Monden, Toyota Production System: Practical Approach to Production Management (Industrial Engineering and Management Press, 1983)