Key cost drivers in spare parts environments
Taken together, spare parts–related costs form a substantial share of the total cost of ownership of assets. While often managed in isolation across maintenance, procurement, warehousing, and finance, these costs ultimately accumulate over the asset lifecycle and determine how capital‑efficient, reliable, and economically sustainable asset operations really are. Spare parts costs are often underestimated because they are fragmented. They do not show up as one line item, but as a collection of operational and financial effects that sit across budgets, functions, and financial statements. The most important cost drivers include:
Cost of spare parts
The most visible cost driver is the purchase price of spare parts themselves. In many organisations, these costs are allocated directly to the maintenance budget and treated as unavoidable operational expenses. This framing is slightly misleading. From an inventory management perspective, these costs are hard to influence, but from a broader perspective, there are opportunities. Setting up contracts with key suppliers can reduce overall costs, while changing maintenance policies affects the number of spare parts needed per year.
Inventory holding costs
Holding spare parts is never free. Typically, we break down holding costs into the three components below, which together account for 20-30% of the total inventory value.
- Interest: capital tied up in inventory cannot be used elsewhere. This is expressed in the opportunity cost of capital.
- Storage costs: warehouses, shelving, handling equipment, and labour all cost money. More spare parts means more storage costs.
- Risk: parts can lose value, become obsolete, get damaged, or simply disappear. Obsolescence is a particularly underestimated driver in long‑life assets.
Order costs
Procurement is not just about unit price. Each purchase order is accompanied by a significant operational workload, e.g., requisitioning, approval, receipt, inspection and payment processing. When replenishing per-part, a simple screw costing $1 can easily multiply its effective cost. Simply put, efficient ordering is essential.
Emergency shipment costs
When stock is insufficient, organisations resort to expedited transport. In aerospace, an AOG order is the classic example. These emergency shipments often cost multiples of standard procurement and introduce additional administrative effort.
Costs related to stockouts
Lastly, but certainly not least importantly, the costs related to stockouts. When a critical part is unavailable, the financial impact can be severe. In asset‑intensive industries, this often translates into downtime costs. In service‑driven businesses, it may result in missed sales or contract penalties. As these costs are often not explicit and invisible until something goes wrong, they are not considered. Yet, they are an important factor to consider in the trade-off between availability and inventory.
