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Cleaning and changeover: cost per batch, not cost per unit

Cleaning between batches costs the same whether the batch was large or small — which is what really makes short runs expensive. How to cost it, and what it tells you about batch sizing.

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Between two batches the plant cleans down. That consumes labor, water, chemicals, energy and — usually the largest part — **production time the line could have been running**.

The important property: it costs almost the same whether the batch before it was large or small. It is a per-batch cost wearing no disguise at all, and yet it is routinely spread as if it were per unit.

What the changeover actually costs

  • Labor for the cleaning crew, for the full duration.
  • Water, chemicals and energy consumed.
  • Effluent treatment for what goes down the drain.
  • Product lost at the start and end of the run — the first and last portions that are not to specification.
  • Line time: the hours the equipment was not producing.
The last item is usually the largest and the most often omitted, because no invoice arrives for it. It is only a real cost if the line was going to run — see the bottleneck guide. On a line with spare capacity it is much smaller than it looks; on a constrained line it is the whole story.

How to put it on the sheet

Total changeover cost divided by the units in the batch that follows. Which means the same product costs different amounts at different batch sizes — and that is not a flaw in the method, it is the fact the method exists to reveal.

Not all changeovers are equal

A changeover between two similar products is short. A changeover that has to remove an allergen, or move between a strongly flavored product and a delicate one, can take far longer and require verification afterwards.

Cost them separately rather than using one average. An average makes easy changeovers look expensive and hard ones look cheap, which is precisely backwards for scheduling decisions.

The question this number answers

**What is the smallest batch worth running?** Below some size, changeover cost per unit exceeds any margin the product carries. Most plants have an opinion about that number; few have calculated it.

Once changeover is costed per batch, the calculation is direct: cost the same product at several batch sizes and read where it stops making money. That is a scheduling rule with a number behind it rather than a habit.

It also settles a recurring argument honestly. Sales want short runs for flexibility; production wants long runs for efficiency. Both are right, and the changeover cost is the exchange rate between them — without it the discussion has no units.

Where changeover cost actually falls

  1. Sequence production so similar products follow each other and hard changeovers are rare.
  2. Prepare everything possible before the line stops, so the stopped time is as short as it can be.
  3. Standardize the cleaning procedure and record it, so it is not re-invented on each shift.
  4. Reconsider batch sizes with the number in hand rather than by habit.

Related guides

Cleaning and changeover: cost per batch, not cost per unit | costdown.org