Spare Parts Stocking Policy: How to Decide What to Stock and What to Buy on Demand

Key takeaways

• Stocking policy = decision framework for which parts to stock, in what quantity, and when to reorder.
• Inputs: asset criticality, part lead time, failure rate, part cost, supplier reliability.
• Critical parts on critical assets with long lead times: always stock.
• Cheap, fast-lead-time, low-criticality parts: order on demand.
• Most plants over-stock low-value items and under-stock the few critical long-lead-time items that actually cause downtime.

Short answer: A spare parts stocking policy is the rule set that decides which parts to stock, in what quantity, and when to reorder. Inputs: asset criticality, supplier lead time, failure rate, part cost. The pattern most plants get wrong: over-stocking low-value items and under-stocking the few critical long-lead-time items that actually cause downtime. A clear policy based on criticality x lead time fixes this. See also Spare Parts Criticality vs Velocity.

What a stocking policy decides

For every part:

• Do we stock it? (Yes / No)
• How many? (Reorder point, target inventory)
• Where? (Central stockroom, line-side, multi-site)
• When to reorder? (Reorder point trigger, supplier lead time)
• Who owns it? (Maintenance, procurement, supplier-managed)

Without a policy, decisions are ad-hoc, inventory drifts, and the wrong parts end up stocked.

The criticality x lead time matrix

Plot every part on two axes:

• Vertical: asset criticality. High = used on a critical asset. Low = used on standard or non-critical asset.
• Horizontal: supplier lead time. Long = weeks or months. Short = days.

Four quadrants emerge:

1. High criticality / Long lead time: always stock. Multiple spares if the part is unique or supplier is unreliable.
2. High criticality / Short lead time: stock a minimum. Reorder on consumption.
3. Low criticality / Long lead time: stock one if cost is reasonable. Buy on demand if expensive.
4. Low criticality / Short lead time: order on demand. No stocking.

Common stocking mistakes

1. Stocking what is easy to stock. Small cheap parts that are easy to order get stocked. Critical expensive long-lead parts get skipped. Inventory looks fine; reality is fragile.

2. No lead time data. Without knowing supplier lead time per part, stocking decisions are guesses.

3. No criticality on parts. Parts are stocked based on familiarity, not asset criticality.

4. Inheriting legacy stocking. Parts have been on the shelf for 10 years. Why? Nobody knows. Some are still relevant; many are not.

5. No min-max or reorder point. Stocking exists but reorder discipline does not. Parts go to zero before reorder.

How to set reorder points

Basic formula:

Reorder point = (Lead time x average usage) + Safety stock

Safety stock accounts for usage variability and supplier reliability. For critical parts, safety stock is meaningful. For non-critical short-lead parts, safety stock is minimal.

Two-bin systems for fast-moving parts

For parts consumed regularly:

• Two physical bins (or virtual partitions) with equal quantity.
• Use bin A until empty; switch to bin B.
• Empty bin A triggers reorder.
• New stock fills bin A; bin B continues to be used.

Simple, visual, hard to forget. Works well for fasteners, lubricants, filters, common consumables.

Vendor-managed inventory for high-volume parts

For parts where the supplier knows your usage:

• Supplier manages the stock at your site.
• Supplier replenishes based on consumption signals.
• You pay on consumption, not on receipt.

VMI works well for high-volume standard parts (bearings, belts, filters). It does not work for one-off critical spares.

The cost of getting it wrong

Under-stocking critical long-lead parts: when the part fails, the asset is down for weeks. Production loss can dwarf inventory savings by 10x or 100x.

Over-stocking low-value parts: working capital tied up, inventory obsolescence, warehouse space consumed. Real cost but rarely catastrophic.

The asymmetry: missing the critical part is much more expensive than over-stocking the non-critical one. Policy should reflect this.

Common mistakes

1. Inventory turnover as the only metric. Optimizing for turnover drives under-stocking of critical parts.

2. No periodic review. Stocking decay if not reviewed annually.

3. Part proliferation. Adding parts without policy. Inventory grows; useful coverage does not.

4. No standardization. Multiple types of the same component (e.g., 6 brands of M8 bolts). Standardize, then stock fewer SKUs.

How a modern CMMS supports stocking policy

A modern CMMS:

• Captures usage per part per asset.
• Tracks supplier lead time per part.
• Calculates reorder points dynamically.
• Triggers reorders automatically.
• Surfaces parts approaching reorder.
• Reports inventory cost vs criticality coverage.

Fabrico's CMMS supports criticality x lead time stocking policy, two-bin systems, dynamic reorder points, and inventory cost vs criticality coverage reporting.

See how Fabrico captures this automatically — explore OEE for manufacturing or book a demo.

Related reading

• Spare Parts Criticality vs Velocity

Frequently asked questions

How often should I review stocking policy?

Annually plus after major equipment changes or supplier changes.

Should I stock OEM parts or aftermarket?

Critical assets: OEM. Standard assets: aftermarket if reliability is comparable. Mixed approach is common.

What inventory turnover is healthy?

Highly dependent on part mix. Total inventory turnover of 2-4x per year is typical; high-value spares may sit for years.

How do I justify a high-cost spare for a critical asset?

Cost of the spare vs cost of one day of downtime. The spare usually wins by orders of magnitude.

Is consignment inventory the same as VMI?

Related. Consignment means the supplier owns the stock until you use it. VMI means the supplier manages replenishment. Often combined.