What Is MRO Inventory (Maintenance, Repair and Operations)?
Also known as: MRO, MRO spares, spare parts inventory, maintenance stores
Definition
MRO inventory is the stock of spare parts, consumables and supplies used to maintain and repair equipment and facilities, as opposed to raw materials and components that become part of a finished product.
MRO Inventory (Maintenance, Repair and Operations) Explained
MRO items include bearings, belts, seals, filters, lubricants, fuses, motors, cutting tool inserts, safety supplies and tools. They are bought for use, not for resale or inclusion in a bill of materials, so they are planned differently from production items. MRP does not drive them from demand. Min and max levels, reorder points and criticality drive replenishment.
Stocking is a risk decision. A cheap item that stops a critical line may justify a stocked spare, while an expensive motor with a short supplier lead time may not. Criticality of the asset, lead time, cost and failure probability inform the choice. Slow-moving critical spares are a classic source of both stockouts and obsolete inventory.
In an ERP, MRO items live in the item master, often flagged as non-production or in a separate store or warehouse. Parts are issued to a work order so cost is charged to the asset. In SAP Plant Maintenance, parts are reserved against maintenance orders and linked through bills of material for equipment. Where a separate CMMS holds the maintenance stores, a common issue is double stock records with different counts.
Clean item data is the main obstacle. The same bearing may exist under five descriptions and three part numbers from different suppliers, producing duplicate stock and missed savings. Cataloging and standardizing before analysis pays back quickly.
Why It Matters
- Missing spares extend downtime, so MRO stock directly affects equipment availability.
- Excess and obsolete spares tie up cash and are often written off at audit.
- Charging parts to work orders is how maintenance cost per asset is calculated.
- Duplicate and poorly described parts hide consolidation savings and cause emergency buys.
In Practice
A plant has 12,000 MRO items and a stores value of 1.8 million. A review finds about 20 percent have had no movement in three years. The team separates them into critical spares that stay (with documented reasons, such as long lead time) and surplus that is returned or sold. They also merge duplicate bearing records, which reveals the same part bought from four suppliers at different prices.
Frequently Asked Questions
How is MRO inventory different from production inventory?
Production inventory goes into products and is driven by MRP from demand. MRO items support equipment and are replenished by min and max levels, reorder points and asset criticality.
What is a critical spare?
A part whose absence would stop a critical asset for a long time and which cannot be sourced quickly, so it is stocked even if it rarely moves.
Should MRO live in the ERP or the CMMS?
Ideally one system owns stock quantities. Many plants keep stock in the ERP for purchasing and finance and let the CMMS read it, or the reverse. Two independent stock records usually drift apart.
Related Terms
CMMS (Computerized Maintenance Management System)
A CMMS (Computerized Maintenance Management System) is software that records equipment, schedules preventive maintenance, issues and tracks maintenance work orders, and manages spare parts, so a maintenance team can plan work and keep a history of every repair.
Reorder Point
A reorder point is the inventory level at which a replenishment order is triggered. It equals expected demand during the replenishment lead time plus safety stock, so stock on hand covers usage until the new supply arrives.
Safety Stock
Safety stock is buffer inventory carried above expected demand to protect against variability in demand and supply during replenishment lead time. In ERP it is netted out of available inventory so MRP plans replenishment before the buffer is consumed.
ABC Analysis
ABC analysis is an inventory classification method that ranks items by annual consumption value and groups them into A, B, and C classes, so planning attention, counting frequency, and control effort are concentrated where the money is.
Cycle Counting
Cycle counting is the practice of continuously verifying inventory accuracy by counting a small subset of items each day or week, rather than shutting down for one annual wall-to-wall physical inventory.
Go Deeper
Spare Parts Inventory Optimization Calculator
Quantify the working capital trapped in spare parts and the annual carrying and stockout savings available from AI-driven inventory optimization.
Free ToolManufacturing Downtime Cost Calculator
Combine lost contribution margin, idle labor, and absorbed overhead into a defensible monthly and annual downtime cost - with recovery effects modeled.
Preventive Maintenance and ERP: Closing the Loop
Preventive maintenance ERP integration guide: CMMS-ERP data flows, meter-based PM triggers, MRO inventory, scheduling impact, and AI-driven predictive maintenance.
SyteLine Fixed Assets Management Guide
Configure fixed asset management in Infor SyteLine including asset creation, depreciation methods, disposal workflows, and book-tax depreciation tracking.
Working with MRO Inventory (Maintenance, Repair and Operations) in a live environment? Our engineers do this every day - and our AI agents automate most of it.