key takeaways
If you only read 30 seconds of this article:
- High total inventory and stockouts coexist because the stock is misallocated: overstocked on parts that rarely fail, understocked on the ones that stop the line.
- The root cause is stocking by cost or usage instead of by criticality and failure risk, so intermittent critical parts get too little.
- Fragmented data hides the truth: the same part under many numbers means you cannot see what you actually have across sites.
- The average asset-intensive manufacturer carries 20-30% excess MRO inventory while facing stockout risk on 10-15% of critical parts, consistent with Verusen's experience across hundreds of implementations.
Spare parts stockouts
Short answer: Spare parts stockouts happen even when inventory is high because the inventory is misallocated, not insufficient. Plants overstock the parts that are easy to forecast and rarely fail, and understock the intermittent, high-consequence parts that actually stop production, because standard formulas stock by demand history that critical parts do not have. Fragmented data across ERPs compounds it by hiding true on-hand. The fix is to stock by criticality and failure risk on unified data, not to carry more.
The MRO inventory paradox: Carrying high total spare-parts inventory while still suffering stockouts on critical parts, because stock is allocated by cost or usage rather than by criticality.
The stock is misallocated, not insufficient
The paradox resolves the moment you stop looking at the total and start looking at the allocation. A full storeroom can be full of the wrong things: sixteen of a pump seal someone over-ordered in a crisis, and zero of the bearing that stops the line. High inventory value and stockouts are not a contradiction; they are the two symptoms of the same misallocation. Purpose-built MRO inventory optimization fixes the allocation rather than the total.
Three causes drive the misallocation, and each is fixable.
| Cause | What happens | Result |
|---|---|---|
| Stocking by cost/usage | Cheap, frequent parts get stocked; rare critical ones do not | Understock on line-stop parts |
| Demand formulas on spare parts | Intermittent parts return near-zero recommended stock | Zero on the part that matters |
| Fragmented data | Same part under many numbers across ERPs | You cannot see true on-hand |
Spare parts stockouts causes
Why more inventory doesn't fix it
The instinct is to buy more, but adding stock on top of a bad allocation just raises the inventory value while leaving the critical gaps open. Standard safety-stock formulas make it worse: they need demand history, and a critical part that fails twice in five years has none, so the formula recommends near-zero and the plant keeps running out of it no matter how full the storeroom gets. The replacement is criticality-based stocking, covered in the companion MRO criticality analysis guide, and the demand-side mechanics for fast movers are in this safety stock formula reference.
Industry estimates put the scale of the misallocation at 20 to 30% excess inventory alongside stockout risk on 10 to 15% of critical parts, consistent with Verusen's experience across hundreds of implementations. This MRO spares optimization guide and this spare parts inventory management guide corroborate the pattern.
| excess MRO inventory carried (industry estimate) | 20-30% |
| critical parts at stockout risk (industry estimate) | 10-15% |
| all-in downtime cost per hour (Aberdeen) | $260K |
Spare parts stockouts fix
How to fix the paradox
Reallocate, do not accumulate. The sequence is to see the truth, then stock to criticality.
- Unify and de-duplicate materials across every ERP so you see true on-hand.
- Re-stock by criticality and failure risk, protecting line-stop parts first.
- Release the excess on rarely-failing parts to fund the critical buffers.
- Pool critical stock across sites so one buffer covers several locations.
- Re-score on a cycle so the allocation stays right as assets change.
Done this way, total inventory falls while availability rises. Most customers reach a working solution in under 45 days, with no data cleanse required first, based on Verusen customer results. Then talk to an MRO expert to see your own allocation.
Further reading: MRO spares inventory optimization guide, spare parts inventory management guide, and safety stock formula methods.
Frequently asked questions
Because the inventory is misallocated, not insufficient. Plants overstock cheap, frequently-used parts and understock the intermittent, high-consequence parts that stop production, so a full storeroom still lacks the one part you need. High inventory value and stockouts are two symptoms of the same misallocation.
No. Adding stock on top of a bad allocation raises inventory value while leaving the critical gaps open. The fix is to stock by criticality and failure risk on unified data, protecting line-stop parts, not to carry more of everything.
Because they need demand history, and a critical part that fails twice in five years has almost none, so the formula recommends near-zero and the plant keeps running out of it. Criticality-based stocking sizes by consequence of failure and lead time instead.
Industry estimates put it at 20 to 30% excess MRO inventory alongside stockout risk on 10 to 15% of critical parts, consistent with Verusen's experience across hundreds of implementations. The two coexist because both come from misallocation.
Reallocate rather than accumulate: unify and de-duplicate materials to see true on-hand, re-stock by criticality, release excess on rarely-failing parts to fund critical buffers, and pool critical stock across sites. Total inventory falls while availability rises, based on Verusen customer results.
PN
- Jeremiah Woodford
- CRO, Verusen
Chief Revenue Officer (CRO) at Verusen AI – AI Built for Industry. Designed to Solve What Legacy Systems Can’t.
