You are probably asking this because your stock levels feel either excessive, unreliable, or difficult to justify.
Deciding how many spare parts to hold is not just a calculation. It is a risk decision.
Hold too few and you increase the likelihood of downtime, urgent freight, maintenance delays, and production disruption.
Hold too many and you tie up cash, consume storage space, increase the risk of obsolescence, and make the inventory harder to manage.
The challenge is that spare parts often have low, irregular, or unpredictable demand.
Standard inventory formulas can be useful, but only when they are applied with an understanding of the operating context.
The common mistake is assuming that the standard inventory formula is suitable for spare parts inventory. It is not.
Intuitively, many organizations understand this and so end up relying on simple rules such as:
“Keep one on the shelf.”
“Hold six months of usage.”
“Use the system recommendation.”
“Set the minimum and maximum based on past demand.”
“Increase the quantity because the supplier lead time is long.”
These rules may look practical, but they often ignore the real issue: determining a single number without understanding the components that make up that number leads to inappropriate stock levels and poor risk management.
A better quantity decision considers both demand and risk.
That means understanding:
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Why splitting out the ‘safety stock’ and ‘cycle stock’ is the key to a better result
That risk is only managed through safety stock
The lead time and lead time reliability
Criticality is separate decision.
The goal is not simply to reduce inventory. The goal is to set stock levels that support operational reliability without creating avoidable excess.
This topic is covered in detail in the Foundation Playbook.
That playbook provides a simple approach to deciding stock levels that can be applied at any time and that delivers the most practical result.
For individuals this playbook is available via the Practitioner Toolkit.
For teams, Foundations for Teams helps create a shared understanding across maintenance, procurement, stores, engineering, and finance — so stocking decisions are not driven by one function alone.
Explore the Practitioner Toolkit
The complete spare parts management toolkit for practitioners.
Enquire about Foundations for Teams
Build a common decision language across the people who influence spare parts outcomes.