You are probably asking this because your inventory has grown over time, parts are still missing when needed, or no one is confident about whether current stock holdings are justified.
Deciding which spare parts to stock is one of the most important decisions in spare parts management — and one of the easiest to get wrong.
Stock too little and you increase the risk of downtime, urgent purchasing, and production disruption.
Stock too much and you lock cash into inventory that may never be used.
The problem is that spare parts do not behave like normal inventory.
They often have low or irregular demand, long and uncertain lead times, and consequences that depend on the equipment, operating context, and availability of alternatives.
That means a stocking decision cannot be made properly using demand history alone.
The most common mistake is treating spare parts like standard inventory items.
In many organisations, the decision to stock a spare part is based on one or more of the following:
“We used one last year.”
“Maintenance says it is critical.”
“The supplier has a long lead time.”
“We have always stocked it.”
“We cannot afford to be caught without it.”
Each of those points may be relevant. But none of them, by itself, is enough to justify a stocking decision.
This is how inventories grow over time. Parts are added because someone is worried, but rarely removed because no one is confident enough to challenge the original decision.
A better stocking decision separates genuine operational need from habit, fear, convenience, and inherited practice.
That means considering questions such as:
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What happens if the part is not available when needed?
How likely is the part to be required?
How long would it take to obtain?
Is there an alternative response if the part is not stocked?
Is this item truly critical, or merely familiar?
Is the stock decision based on current operating reality or past assumptions?
The objective is not to stock less for the sake of reducing inventory.
The objective is to stock the right parts, for the right reasons, with a decision process that can be explained and defended.
This topic is covered in the Foundation Playbook, which addresses the core principles behind spare parts stocking decisions and why standard inventory logic often fails. We provide clear direction, including 7 questions to ask and how to use the ‘planning horizon’ technique.
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.