MRO and Spare Parts Inventory Statistics: Stockouts, Accuracy, Costs

MRO Inventory Statistics at a Glance

Why MRO Inventory Statistics Are Hard to Pin Down

Start with the structural problem. Production inventory shows up in government data: the Census Bureau’s M3 survey tracks manufacturers’ inventories monthly, with materials and supplies, work in process, and finished goods each getting a line. MRO inventory has no dedicated category. Spare parts, consumables, and maintenance supplies are folded into other accounts, capitalized inconsistently from one company to the next, and we could not locate any public agency or standards body that surveys them as a benchmark category.

That vacuum gets filled by three kinds of sources. Peer-reviewed research exists, but the large-scale empirical studies of inventory record accuracy were conducted in retail, where auditing hundreds of thousands of records is feasible. Consultant and vendor estimates fill the MRO-specific gap, and they are often drawn from the client bases of firms selling inventory optimization services, a sample that skews toward organizations with known inventory problems. Finally, a small number of named industry surveys with disclosed sample sizes have appeared in recent years, and these are the closest thing the field has to benchmark data.

This page separates those three tiers. Where a figure comes from a vendor with a commercial stake in the number, that stake is stated.

Stockout Rates and the Downtime Connection

What the surveys actually measured

The most specific recent data point comes from SPARETECH’s survey of 300 manufacturing executives across the United States and the DACH region, published in its report The MRO Strategy Gap. In that survey, 32 percent of operations leaders reported frequent stockouts of critical spare parts. In the same survey, 48 percent of procurement and supply chain executives named high inventory spend as a top challenge, rising to 61 percent among US respondents.

The two findings side by side describe the standard MRO paradox: plants that are simultaneously overstocked and underprepared, usually because nobody can see what is actually on the shelf across sites. The report also found that 45 percent of executives cited lack of inventory visibility across sites as their top MRO challenge, ranking it above downtime, obsolete stock, and workforce constraints.

One caveat applies to all of these figures: SPARETECH sells spare parts data and inventory software, and vendor surveys tend to frame questions around the problems their products address. The sample size and geography are disclosed, which puts this survey ahead of most claims in the category, but it remains a vendor survey rather than independent research.

The “50 percent of downtime” claim, traced

The single most-quoted stockout statistic in vendor marketing reads something like this: “According to Aberdeen Group, 50 percent of annual unscheduled asset downtime can be attributed to the lack of spare parts and stockouts.”

Trace it back and the trail ends at a June 2018 IBM press release announcing the acquisition of Oniqua, an MRO inventory optimization company. The press release attributes the figure to Aberdeen Group without naming a report, a year, a sample, or a methodology. No Aberdeen publication containing the figure is publicly locatable. Since 2018 the claim has been reproduced across dozens of vendor sites, sometimes as “up to 50 percent,” and IBM’s own marketing for the acquired product now cites “up to 50 percent reduction in unplanned downtime related to parts” as a customer outcome, a different claim that appears to have cross-pollinated with the original.

To complicate matters, a competing version circulates: some vendor content states that 23 percent of unplanned downtime events are caused by unavailable spare parts, attributed in one article to Aberdeen Group and in another article on the same site to a Plant Engineering maintenance survey. Conflicting attribution for the same figure is usually a sign that the number is being passed between secondary sources rather than pulled from a primary one.

The honest summary: parts unavailability is a real and well-documented driver of extended downtime, but no traceable study supports a specific percentage of downtime attributable to stockouts. Treat any such percentage as marketing until its author produces the report.

What downtime costs when the part is missing

The downtime side of the equation has better documentation, with limits worth stating. The Siemens/Senseye report The True Cost of Downtime 2024 estimated that unplanned downtime costs Fortune Global 500 companies roughly $1.4 trillion per year, about 11 percent of revenues, up from an estimated $864 billion (8 percent of revenues) in the 2019 to 2020 edition. The report puts an idle automotive production line at up to $2.3 million per hour.

Siemens sells predictive maintenance software, and the headline figure is an extrapolation rather than a measured census: it rests on 181 completed online interviews with maintenance, engineering, and IT professionals at large industrial organizations across automotive, FMCG, heavy industry, and oil and gas, combined with an assumption that maintenance spend runs about 3 percent of turnover across sectors. It is, however, a named report with a published methodology, which is more than most downtime figures can claim. For a fuller breakdown of downtime cost data, see our guides to the cost of unplanned downtime in manufacturing and unplanned downtime frequency benchmarks.

The practical link between stockouts and downtime cost is lead time. A condition monitoring program can flag a bearing 30 days before failure, but if the replacement has a six-week supplier lead time and the storeroom shelf is empty, the warning buys nothing except a choice between premium expediting and running to failure.

Inventory Record Accuracy

The 65 percent figure and where it actually comes from

When vendor content claims that “most inventory records are wrong,” the number behind the claim is almost always 65 percent, and it comes from a real study: DeHoratius and Raman, “Inventory Record Inaccuracy: An Empirical Analysis,” published in Management Science in 2008. The researchers examined nearly 370,000 inventory records across 37 stores of a single US retailer and found 65 percent of records did not match the physical count. The average absolute gap was about five units per SKU, roughly 35 percent of the average quantity on the shelf.

The study is rigorous and heavily cited, and it is about retail stores. It says nothing directly about maintenance storerooms. Related retail findings cluster in the same range: Kang and Gershwin (2005) reported average record accuracy of 51 percent at a global retailer, and a large European study by Rekik, Syntetos, and Glock for ECR Retail Loss (2019), covering roughly one million SKUs across about 100 stores of seven European retailers, found about 60 percent of SKUs affected by record inaccuracies, with average discrepancies of roughly +6.6 and -6.0 units. The same ECR study found that correcting the records lifted sales by 4 to 8 percent, which gives some sense of what inaccuracy costs even in a setting with far more forgiving demand patterns than a storeroom.

We could not locate a comparable large-scale, peer-reviewed audit of MRO storeroom records. Storerooms differ from retail in ways that cut both directions: they have no customer theft and far fewer transactions, which should help accuracy, but they also have open storerooms with uncontrolled access, parts issued without work order transactions, and technicians hoarding critical spares in toolboxes, which hurt it.

Practitioners who run cycle count programs in storerooms commonly target 95 to 98 percent record accuracy as a working standard, but that is a management target rather than a measured industry average, and no primary survey establishes what typical storeroom accuracy actually is.

Why record accuracy is the upstream statistic

Record accuracy sits upstream of both stockout and carrying cost problems, which is why it belongs in the middle of this page. A record that overstates on-hand quantity suppresses the reorder trigger and produces a stockout with a clean-looking system. A record that understates quantity triggers a purchase of a part already on the shelf. The SPARETECH survey found roughly 66 percent of manufacturers manage MRO with no dedicated system beyond their ERP, and only about 8 percent describe their spare parts management as fully standardized across sites.

One additional piece of vendor research points the same direction. Verdantis, an MRO master data vendor, reported from its own research with roughly 1,900 senior operations executives that 51 percent cited data quality issues in MRO operations and that duplicate purchases attributable to poor data run 5 to 7 percent of spend. As with the SPARETECH figures, the vendor sells products that address exactly these problems, so read the numbers as directionally consistent with the visibility findings above rather than as independent confirmation.

Carrying Costs

The traceable origin of the standard range

Nearly every carrying cost percentage quoted in inventory content descends from one source: Helen Richardson’s article “Control Your Costs Then Cut Them,” published in Transportation and Distribution in December 1995. Richardson estimated total annual inventory carrying cost at 25 to 55 percent of inventory value, broken down as follows:

Three decades later, this table is still what consultants and vendors are paraphrasing when they say carrying costs run “20 to 30 percent” or “at least 25 percent” of inventory value. The figure was a trade press estimate when it was published, the component breakdown now circulates mostly through secondary reproductions of the original table, and the cost of money component in particular has swung dramatically since 1995 as interest rates have moved. The commonly repeated rule of thumb of 25 percent appears in logistics textbooks of similar vintage.

APQC publishes a broad inventory carrying cost measure through its open-standards benchmarking program, but not a public MRO-specific benchmark in the 15 to 30 percent range commonly attributed to it, and ASCM (formerly APICS) covers carrying cost in its certification body of knowledge without a public primary document behind the quoted ranges either. Until someone produces those documents, the traceable published anchor for carrying cost percentages remains a 1995 magazine article, which says a great deal about the state of benchmarking in this field.

What carrying cost means for a storeroom

Applied to MRO, the arithmetic is blunt. A plant holding $10 million in spare parts at a conservative 25 percent carrying rate spends $2.5 million per year to own that inventory before a single part is used. The obsolescence component hits MRO harder than production inventory because spare parts demand is intermittent and tied to specific installed equipment: when the asset is retired, its dedicated spares become scrap at book value. This is the mechanism behind the unused inventory figures in the next section, and it is why carrying cost percentages for slow-moving spares are plausibly at the high end of any quoted range.

Some practitioners benchmark total stocked MRO inventory value against replacement asset value (RAV), and specific target percentages circulate in vendor and consultant content. We could not verify a public primary source behind any universal RAV-based stocking target, so this page does not repeat one. Asset criticality, supplier lead time, and consequence of failure are more defensible stocking inputs than a ratio, and any RAV-denominated comparison inherits the definitional problems of RAV itself on one side and the completeness of the inventory count, including consignment and vendor-managed stock, on the other.

Excess, Obsolete, and Inactive Inventory

The “50 to 60 percent” claim, traced

The second great unkillable MRO statistic: “50 to 60 percent of MRO inventory is excess, obsolete, or slow-moving.” Depending on the vendor page, this is attributed to GEP, to Oniqua, to IBM, to McKinsey, or to unnamed “studies.” Versions of the claim appear across consultant and vendor content, but none we found led to a published study with a sample and a method. The oldest versions read as consultant estimates drawn from client engagements, and consultant client bases are self-selecting: companies that hire inventory rationalization consultants tend to be the ones with inventory problems.

The underlying phenomenon of bloated storerooms is real and widely observed. The specific percentage, however, is unverifiable as an industry average.

The figure that is actually documented

The defensible modern number is narrower and better sourced: the SPARETECH survey of 300 manufacturing executives found that 22 percent of MRO inventory, on average, has gone unused for more than five years. Five years without a single issue transaction is a strong signal that a part will never be used, though not proof: insurance spares for critical equipment are stocked precisely in the hope they are never issued, and a slow-moving critical spare is a deliberate risk decision rather than a management failure.

Any obsolescence analysis that does not first separate criticality-justified insurance spares from genuinely dead stock will overstate the problem, which is one reason consultant-reported inactive percentages run so high.

The same survey offers the working capital counterpoint: among the minority of manufacturers using dedicated MRO software, 96 percent reported inventory reductions, averaging 9 percent. That figure comes from the software vendor conducting the survey, so it belongs in the same drawer as every vendor ROI claim, though the result is directionally plausible.

How to Read MRO Inventory Statistics

Some patterns recur so consistently in this content vein that they work as a checklist:

A percentage with a shifting attribution is a secondhand number. When the same figure is credited to Aberdeen on one page, Plant Engineering on another, and GEP on a third, no one quoting it has read a primary source.

A press release is not a study. The most-circulated stockout statistic in the industry entered circulation through an acquisition announcement.

Vendor surveys with disclosed samples beat vendor claims without them. A survey of 300 named-role respondents with stated geography can be weighed; “studies suggest” cannot.

Retail research is not storeroom research. The best inventory record accuracy studies ever conducted took place in stores. Their findings are suggestive for MRO and are routinely quoted as if they were MRO data. They are not.

Old numbers do not age out on their own. The standard carrying cost range predates the modern ERP era, and it will keep being quoted until something better replaces it. Little has replaced it publicly.

FAQ

What percentage of MRO inventory is obsolete or inactive?

The widely quoted claim that 50 to 60 percent of MRO inventory is excess, obsolete, or slow-moving has no locatable primary study behind it; it traces to consultant estimates drawn from self-selected client engagements. The best-documented recent figure comes from a 2025 survey of 300 manufacturing executives conducted by SPARETECH, an MRO software vendor, which found 22 percent of MRO inventory on average has gone unused for more than five years. Note that some long-idle stock consists of deliberately held insurance spares for critical equipment, so unused is not identical to unjustified.

How accurate are MRO inventory records?

We could not locate a large-scale published audit of MRO storeroom record accuracy. The figures usually quoted come from retail research: DeHoratius and Raman’s 2008 Management Science study found 65 percent of roughly 370,000 records at one US retailer were inaccurate, and Kang and Gershwin reported average accuracy of 51 percent at a global retailer. Storeroom practitioners commonly target 95 to 98 percent record accuracy through cycle counting, but that is a management target rather than a measured industry average.

What does it cost to carry MRO inventory?

The standard range of 25 to 55 percent of inventory value per year is cited back to Helen Richardson’s 1995 article in Transportation and Distribution, which broke the total into cost of money, taxes, insurance, warehousing, handling, clerical work, obsolescence, and deterioration. The lower ranges commonly quoted today, typically 15 to 30 percent, circulate through vendor content without a verifiable primary benchmark behind them. For spare parts specifically, obsolescence risk pushes carrying cost toward the high end of any quoted range because demand is intermittent and tied to specific installed assets.

Do stockouts really cause 50 percent of unplanned downtime?

No traceable study supports that figure. It entered circulation through a 2018 IBM press release attributing it to Aberdeen Group without naming a report, and no Aberdeen publication containing it is publicly locatable. A competing 23 percent version circulates with conflicting attributions. Parts unavailability genuinely extends downtime, particularly for long-lead-time components, but any specific percentage of downtime attributed to stockouts should be treated as unverified.

What is a good stockout rate for MRO inventory?

There is no independent benchmark study establishing typical industry performance or an ideal rate. Vendor guidance commonly suggests keeping stockouts below 1 percent of total items picked, calculated as stockout events divided by total pick requests, but that target is practitioner convention rather than research. Most reliability organizations set service level targets by asset criticality rather than applying one universal stockout rate across the whole storeroom, accepting deliberate stockout risk on non-critical items to fund availability on critical ones.

How much MRO inventory should a plant hold?

Some practitioners benchmark stocked MRO inventory value as a percentage of replacement asset value, but we could not verify a public primary source behind any universal target percentage. The more defensible approach works from asset criticality: stock levels for critical spares are set by failure consequence and supplier lead time, while non-critical consumables are managed on min/max or vendor-managed arrangements. Benchmarks denominated in RAV inherit all the definitional problems of RAV itself, so comparisons between plants require matching definitions on both sides of the ratio.

Sources and References

• DeHoratius, N. and Raman, A. “Inventory Record Inaccuracy: An Empirical Analysis.” Management Science, Vol. 54, No. 4 (2008), pp. 627-641. pubsonline.informs.org
• Kang, Y. and Gershwin, S. B. “Information Inaccuracy in Inventory Systems: Stock Loss and Stockout.” IIE Transactions, Vol. 37, No. 9 (2005), pp. 843-859. DOI: 10.1080/07408170590969861
• Rekik, Y., Syntetos, A., and Glock, C. “Inventory Inaccuracy in Retailing: Does It Matter?” ECR Retail Loss research report (2019). Full report (PDF)
• ECR Retail Loss. “Measuring the Sales Impact of Improving Inventory Records.” ecrloss.com
• Richardson, H. “Control Your Costs Then Cut Them.” Transportation and Distribution, December 1995, pp. 94-96.
• SPARETECH. “The MRO Strategy Gap: The Overlooked Lever for Margin Growth.” Survey of 300 manufacturing executives, US and DACH region (2025). sparetech.io
• SPARETECH. “The MRO Paradox: How Manufacturers Are Overspending Their Way into Downtime.” sparetech.io (48 percent and 61 percent inventory spend, 32 percent stockout figures)
• SPARETECH. “You Can’t Manage Spare Parts You Can’t See.” sparetech.io (45 percent visibility figure)
• SPARETECH. “Front-End Spare Parts Decisions That Quietly Break Your MRO Strategy.” sparetech.io (22 percent unused and 66 percent no-dedicated-system figures)
• SPARETECH. “How MRO Software Boosts Manufacturing Margins.” sparetech.io (96 percent and 9 percent inventory reduction figures)
• Siemens/Senseye. “The True Cost of Downtime 2024.” Full report (PDF)
• Siemens/Senseye. “The True Cost of Downtime 2022.” Full report (PDF)
• IBM. “IBM Acquires Oniqua Holdings Pty Ltd.” Press release, June 2018 (origin point of the Aberdeen 50 percent downtime attribution).
• Verdantis. “MRO Data and Maintenance Statistics.” Research with approximately 1,900 senior operations executives (2026). verdantis.com
• APQC. “Inventory Carrying Cost as a Percentage of Inventory Value.” Open Standards Benchmarking measure. apqc.org