Spare Parts Inventory KPIs: 12 Metrics with Formulas
You can’t improve a storeroom you don’t measure. Here are 12 spare parts inventory KPIs, grouped by what they tell you, each with a formula, a worked example, and what to watch for, plus which ones to start with and how to set targets that fit your operation.
The 12 KPIs at a Glance
Spare parts inventory KPIs fall into four groups. Availability KPIs show whether parts are there when needed. Accuracy KPIs show whether the records can be trusted. Cost KPIs show whether the storeroom is carrying the right amount. Process KPIs show whether purchasing and suppliers are keeping up.
| KPI | Group | Formula |
|---|---|---|
| Stock-Out Rate | Availability | (Work orders delayed by a missing part ÷ Work orders that needed parts) × 100 |
| Fill Rate | Availability | (Parts requests filled immediately from stock ÷ Total parts requests) × 100 |
| Downtime Waiting for Parts | Availability | Total equipment downtime hours where the cause was a missing or late part |
| Inventory Record Accuracy | Accuracy | (Counts matching the system record ÷ Total counts) × 100 |
| Work Order Issue Compliance | Accuracy | (Parts issues recorded against a work order ÷ Total parts issues) × 100 |
| Inventory Turns | Cost | Annual value of parts issued ÷ Average inventory value |
| Days of Inventory on Hand | Cost | 365 ÷ Inventory turns |
| Carrying Cost | Cost | Average inventory value × Annual carrying cost rate |
| Dead Stock Percentage | Cost | (Value of parts with no usage in 24 months ÷ Total inventory value) × 100 |
| Inventory Value as % of Asset Value | Cost | (Total MRO inventory value ÷ Replacement value of the equipment it supports) × 100 |
| Emergency Order Rate | Process | (Rush or expedited purchase orders ÷ Total purchase orders) × 100 |
| Supplier On-Time Delivery | Process | (Orders received by the promised date ÷ Total orders received) × 100 |
All examples below use the same sample storeroom, so the numbers connect: 600 work orders a month needing parts, about $400,000 in average inventory, and $600,000 of parts issued per year. They’re illustrations, not industry benchmarks.
Availability KPIs
Is the part there when a technician needs it?
1. Stock-Out Rate
Formula: (Work orders delayed by a missing part ÷ Work orders that needed parts) × 100
Example: 600 work orders needed parts last month and 18 were delayed waiting on a part: 18 ÷ 600 × 100 = 3%.
The most direct measure of whether the storeroom is failing technicians. Track which parts caused each stock-out.
2. Fill Rate
Formula: (Parts requests filled immediately from stock ÷ Total parts requests) × 100
Example: Of 600 requests, 570 were filled on the spot: 570 ÷ 600 × 100 = 95%.
The mirror of stock-out rate, measured at the counter. A falling fill rate is an early warning before stock-outs start delaying jobs.
3. Downtime Waiting for Parts
Formula: Total equipment downtime hours where the cause was a missing or late part
Example: Three breakdowns this month waited 4, 6, and 4 hours for parts: 14 hours.
Ties the storeroom to production. Multiply by your downtime cost per hour to show the business impact.
Accuracy KPIs
Can the inventory records be trusted?
4. Inventory Record Accuracy
Formula: (Counts matching the system record ÷ Total counts) × 100
Example: 12 of 13 parts counted today matched their records: 12 ÷ 13 × 100 = 92.3%.
If the records are wrong, every other KPI is wrong too. Measured through cycle counting.
5. Work Order Issue Compliance
Formula: (Parts issues recorded against a work order ÷ Total parts issues) × 100
Example: 940 of 1,000 issues last month were tied to a work order: 940 ÷ 1,000 × 100 = 94%.
Parts issued without a work order don’t build cost or usage history for the asset, and they’re a common source of count errors.
Cost KPIs
Is the storeroom carrying the right amount of stock?
6. Inventory Turns
Formula: Annual value of parts issued ÷ Average inventory value
Example: $600,000 of parts issued over the year, with $400,000 average inventory: 600,000 ÷ 400,000 = 1.5 turns.
MRO inventory turns slower than production inventory because critical spares sit by design. Watch the trend and the reasons, not just the number.
7. Days of Inventory on Hand
Formula: 365 ÷ Inventory turns
Example: 365 ÷ 1.5 = about 243 days of inventory on hand.
The same information as turns, in a form that’s easier to explain: how long current stock would last at the current usage rate.
8. Carrying Cost
Formula: Average inventory value × Annual carrying cost rate
Example: $400,000 average inventory × an example carrying rate of 20% = $80,000 per year.
Makes the cost of excess stock visible. Your finance team can supply your actual carrying cost rate.
9. Dead Stock Percentage
Formula: (Value of parts with no usage in 24 months ÷ Total inventory value) × 100
Example: $60,000 of parts haven’t moved in 24 months, out of $400,000: 60,000 ÷ 400,000 × 100 = 15%.
Separate true dead stock from critical and insurance spares, which are meant to sit on the shelf.
10. Inventory Value as % of Asset Value
Formula: (Total MRO inventory value ÷ Replacement value of the equipment it supports) × 100
Example: $400,000 of inventory supporting $50 million of equipment: 400,000 ÷ 50,000,000 × 100 = 0.8%.
Puts inventory size in context of the plant it supports, which is more useful than comparing raw dollar amounts between sites.
Process KPIs
Are purchasing and suppliers keeping up?
11. Emergency Order Rate
Formula: (Rush or expedited purchase orders ÷ Total purchase orders) × 100
Example: 12 of 150 purchase orders last month were expedited: 12 ÷ 150 × 100 = 8%.
Emergency orders cost more and usually point to min/max levels that are set too low or not enforced.
12. Supplier On-Time Delivery
Formula: (Orders received by the promised date ÷ Total orders received) × 100
Example: 135 of 150 orders arrived on time: 135 ÷ 150 × 100 = 90%.
Late suppliers force higher safety stock. Track by vendor to see which ones are driving it.
Which KPIs to Start With
Tracking all 12 from day one is more than most teams can keep up with. If you’re starting from scratch, begin with these four:
- Stock-out rate, because it’s what technicians feel.
- Inventory record accuracy, because every other number depends on it.
- Inventory turns, because it shows whether you’re overstocked.
- Emergency order rate, because it shows whether your stocking rules are working.
Add the others once these four are measured consistently every month. The MRO storeroom best practices guide covers the day-to-day habits that move these numbers.
How to Set Targets
Published benchmarks for MRO inventory vary widely by industry, plant size, and how many critical spares a site carries, so a number that’s right for one plant can be wrong for another. A more reliable approach:
- Measure a baseline first. Track each KPI for at least three months before setting a target.
- Set targets as improvements. “Cut stock-outs from 3% to 2% this year” is more useful than an industry number that may not fit your plant.
- Watch pairs of KPIs together. Stock-outs can always be lowered by buying more inventory. The goal is fewer stock-outs without turns falling or carrying cost rising.
- Review monthly, act quarterly. Month-to-month numbers bounce around. Look for trends before changing stocking rules.
Common Mistakes Tracking Inventory KPIs
- Trusting KPIs built on bad records. If inventory accuracy is low, turns, dead stock, and stock-out numbers are all unreliable. Fix accuracy first.
- Counting critical spares as dead stock. Parts held on purpose for critical equipment will always look slow-moving. Report them separately.
- Chasing turns at the expense of uptime. Cutting inventory to improve turns can raise stock-outs and downtime, which cost far more.
- Tracking without acting. Each KPI should lead to a decision: adjust a min/max level, clear dead stock, or talk to a late supplier.
How a CMMS Makes Inventory KPIs Possible
Every KPI above depends on transaction data: what was issued, received, ordered, counted, and which work order it belonged to. Collecting that by hand is what keeps most teams from measuring inventory at all.
Transactions recorded as they happen. Parts issued on work orders and received against purchase orders give you usage, turns, and issue compliance without separate tracking.
Work order history. Delays and downtime recorded on work orders show where missing parts actually hurt production.
Counts in the same system. Cycle count results feed inventory record accuracy directly.
Reports instead of spreadsheets. With the data in one place, KPIs can be reviewed monthly without anyone rebuilding them by hand.
Frequently Asked Questions
What are the most important spare parts inventory KPIs?
Start with stock-out rate, inventory record accuracy, inventory turns, and emergency order rate. Together they show whether parts are available, whether the records are trustworthy, whether you’re overstocked, and whether your stocking rules are working.
How do you calculate inventory turns for MRO parts?
Divide the annual value of parts issued by the average inventory value. For example, $600,000 of parts issued in a year with $400,000 of average inventory gives 1.5 turns. MRO inventory typically turns more slowly than production inventory because critical spares are held on purpose.
What’s the difference between stock-out rate and fill rate?
Fill rate measures how often a parts request is filled immediately from stock. Stock-out rate measures how often work orders are actually delayed by a missing part. Fill rate is an earlier warning; stock-out rate shows the impact on maintenance work.
What is a good inventory record accuracy for a maintenance storeroom?
It depends on your starting point and how critical your spares are. Measure your baseline through cycle counting, then set improvement targets. Accuracy matters most for critical and high-value parts, so many teams track it separately for those.
Conclusion
Spare parts inventory KPIs turn the storeroom from something people complain about into something you can manage. Start with a few, get the records accurate, measure a baseline, and watch availability and cost together, so improvements in one don’t come at the expense of the other.
Measure Your Storeroom Without Rebuilding Spreadsheets
Inventory KPIs are only as good as the transactions behind them. Recording every issue, receipt, and count in one place is the job of spare parts and inventory management in a CMMS. eWorkOrders has supported maintenance operations for over 30 years, serving teams at Honda and King River Packaging, rated 4.9 on Capterra and G2.
About the author: Janet Jaquis is Marketing Director at eWorkOrders, where she writes about maintenance strategy, asset reliability, and CMMS software for maintenance and facility teams.
