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How to Measure OEE and Track Downtime Honestly
OEE, overall equipment effectiveness, is availability times performance times quality: the share of planned time a machine spends making good parts at its ideal rate. Its value comes from the losses it exposes, not the headline number. Track downtime automatically from machine states, have operators add reasons, and fix the largest loss first.
Published · Last reviewed by Geek Engineering senior engineering review.
The calculation
| Factor | Formula | Loss it exposes |
|---|---|---|
| Availability | Run time ÷ planned production time | Breakdowns, changeovers, waiting for material |
| Performance | (Ideal cycle time × total count) ÷ run time | Slow cycles and short stops |
| Quality | Good count ÷ total count | Scrap and rework |
| OEE | Availability × Performance × Quality | All of the above combined |
How OEE numbers get inflated
A widely quoted 85 percent 'world-class' figure is not a useful target across different processes. Compare a line with itself over time, using consistent definitions.
- Using a slow ideal cycle time, so performance looks high
- Excluding changeovers or breaks from planned time without saying so
- Counting rework that eventually passes as good first-time output
- Recording downtime by hand at the end of the shift, from memory
Setting up downtime tracking
- Capture machine states automatically from the PLC: running, stopped, faulted, starved, blocked.
- Give operators a short list of reason codes, ten to fifteen at most, grouped by category.
- Ask for reasons only on stops longer than a set threshold, so short stops are counted but not annotated.
- Review the top losses with the team every week and assign actions.
- Revise reason codes after a month, once you see which ones are used and which are dumping grounds.
Where the data lives
Downtime tracking can run in SCADA, in a dedicated OEE product or in an MES. Choose based on scale and whether the data must later link to work orders, quality records and maintenance systems. Even a simple system pays back when it replaces whiteboards and memory with accurate stop data.
Key takeaways
- OEE is availability × performance × quality.
- Its value is the losses it exposes, not the headline percentage.
- Capture stops automatically and keep reason codes short.
- Compare a line with itself, with consistent definitions.
Related questions
What is a good OEE score?
It depends on the process, product mix and how planned time is defined. A high-volume single-product line and a high-mix job shop are not comparable. The useful question is whether your OEE is improving under consistent definitions.
What is the difference between OEE and TEEP?
OEE measures effectiveness during planned production time. TEEP, total effective equipment performance, measures against all calendar time, so it also shows the capacity left unused by unscheduled shifts.
Can OEE be tracked on older machines?
Yes. If the PLC cannot provide states, simple sensors or an I/O module can detect running, stopped and part counts, and an edge device can send them to the tracking system.
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