Many factories run on waves. One week the plant is idle waiting for orders; the next, everyone works overtime to meet a big shipment. Large batches of one product are run because the changeover is painful, so other products wait and stock piles up. Suppliers see erratic orders and pad their lead times. Quality slips during the rush. Heijunka — production levelling — breaks this cycle by smoothing what the factory produces over time.
The cost of uneven production
- Inventory — large batches create stock that sits for weeks.
- Overtime and idle time — capacity is sized for peaks and wasted in troughs.
- Long lead times — products wait for their turn in the batch cycle.
- Quality problems — rushing during peaks, and defects discovered late in large batches.
- Supplier problems — erratic orders lead to shortages and premium freight.
- Stress — people, machines and planners are always reacting.
Two kinds of levelling
Volume levelling
Produce roughly the same total quantity each day or week, based on average demand over a period, rather than following every spike in orders. A small finished goods buffer absorbs short-term variation.
Mix levelling
Instead of producing all of product A on Monday, all of B on Tuesday and all of C on Wednesday, produce some A, some B and some C every day — or in an even smaller repeating pattern. Each product is available more often, and inventory of each falls.
A simple example
A plant makes three products with weekly demand of 500 A, 300 B and 200 C. Producing in weekly batches means the plant switches products only a few times a week, and each product has up to a week of stock. A levelled daily schedule would make 100 A, 60 B and 40 C every day. Stock of each product drops to about a day, and every product is available every day — but it requires several changeovers daily instead of a few per week.
Changeovers come first
Mix levelling increases the number of changeovers. If a changeover takes several hours, frequent switching wastes too much capacity. That's why levelling usually goes hand in hand with changeover reduction (SMED). As changeover time falls, the plant can afford to switch more often and level more finely.
Every product every interval
A useful concept is EPEI — every product every interval. If the plant can currently produce every product once every two weeks, the goal might be every week, then every day. Calculate the time available for changeovers after production time, divide by changeover time, and you know how many changeovers are affordable — and therefore how short the cycle can be.
Where to level
Levelling is applied at the pacemaker process — usually the final assembly or packing step that receives the customer schedule. Upstream processes are then linked by kanban or FIFO lanes, so they also see smooth demand. If every department is scheduled separately, levelling at one point won't spread through the plant.
Heijunka boxes
A traditional tool is the heijunka box: a board with slots for time intervals across the day and rows for products. Kanban cards placed in the slots show what to produce in each interval, in a levelled sequence. Material handlers collect cards at each interval and deliver them to the pacemaker. The same idea works digitally — a levelled sequence shown on a screen at the line.
Handling uneven customer demand
- High-volume, repeating products — make to a levelled schedule and serve customers from a small finished goods buffer.
- Low-volume products — produce in a fixed slot in the cycle (for example, every Thursday), or make to order.
- Custom products — make to order, but reserve capacity for them in the levelled plan.
See also our guide to make-to-order versus make-to-stock.
Starting small
- Analyse a few months of demand by product to find average daily volumes and variation.
- Pick a product family and its pacemaker process.
- Reduce changeover time on the pacemaker.
- Set a levelled daily volume and a repeating product cycle.
- Size a small finished goods buffer for high-volume products.
- Link upstream processes with kanban or FIFO.
- Review daily adherence and adjust levels monthly as demand changes.
An example
A manufacturer of plastic housings ran each mould for a week at a time because changeovers took most of a shift. Stock was high, yet stockouts were frequent for products waiting their turn. After SMED work reduced changeovers to well under an hour, the plant moved to a two-day cycle for the main products, held a small buffer for the top sellers and scheduled only the moulding pacemaker. Finished goods stock fell substantially, stockouts dropped and overtime became rare.
Common mistakes
- Trying to level mix before reducing changeover time.
- Following every order spike with the schedule.
- Scheduling every department independently.
- No finished goods buffer for high runners.
- Setting levels once and never reviewing them.
How software helps
Production planning software can calculate levelled volumes from demand history, build repeating product cycles, schedule the pacemaker, trigger upstream replenishment and show adherence in real time. See production planning software.
Talk to us about smoothing your production schedule.
Frequently asked questions
What does heijunka mean?
It's a Japanese term meaning levelling. In manufacturing it means smoothing production volume and product mix over time instead of producing in large, irregular batches.
Does levelling mean producing the same thing every day?
It means producing a similar total volume and a similar mix of products in each period, so the process and suppliers see steady demand.
Why does changeover time matter for levelling?
Levelling mix requires more frequent changeovers. If changeovers are long, frequent switching is expensive, so changeover reduction usually comes first or alongside.
Can we level if customer orders are very uneven?
Partly. A small finished goods buffer for high-volume items can absorb variation, letting production stay level. Low-volume or custom items may still be made to order.
Is heijunka only for large automotive plants?
No. Small manufacturers can apply the principles — steady daily volume, a repeating product cycle, small buffers — without special equipment.
