RESOURCES · DEFINITIONS
How to calculate OEE — and how to explain it.
Availability × Performance × Quality. The formula takes a minute; every vendor publishes it. The part nobody helps you with is the next question — why the number moved — because that answer is never in the same system as the number.
THE FORMULA Three rates, one number.
OEE — Overall Equipment Effectiveness — is the product of three rates measured over the same window: Availability × Performance × Quality. Each is a ratio of what actually happened to what could have happened, and each fails for different reasons, which is exactly why the product alone never tells you what to fix.
- Availabilityrun time ÷ planned production time. Breakdowns, changeovers, setup, waiting on a part or an operator.
- Performanceactual output ÷ theoretical output at nominal rate. Micro-stops, reduced speed, a machine nursed along.
- Qualitygood parts ÷ total parts. Scrap, rework, start-up rejects after a changeover.
THE HONEST PART The number is only as good as the window.
Most OEE arguments are really arguments about the denominator. Is a planned changeover excluded from availability, or counted against it? Is a 90-second micro-stop a stop or a performance loss? Is a lunch break part of planned production time? None of these have a universal answer — but a number computed one way and compared against a number computed the other way is worthless. Write the convention down once, and make every chart cite it.
THE DIAGNOSIS Six big losses, three owners.
The classic decomposition splits OEE into six losses: breakdowns and setup/adjustment (availability), idling/micro-stops and reduced speed (performance), process defects and start-up rejects (quality). What the framework does not say out loud is that the evidence for each one lives with a different team — maintenance owns the breakdown history, production owns the rate, quality owns the rejects.
| Loss | Where the evidence actually sits |
|---|---|
| Breakdowns | CMMS work orders and failure history |
| Setup & adjustment | MES / production orders, changeover records |
| Micro-stops | SCADA tags and alarm logs — often never aggregated |
| Reduced speed | Historian trends against nominal rate |
| Process defects | MES quality events, batch records |
| Start-up rejects | MES, tied to the changeover that preceded them |
This is why “why did OEE drop?” takes three reports and a meeting.
WHAT ACTUALLY HELPS From a number to a cause.
A dashboard can tell you availability fell four points on Line 4 last week. Answering why means crossing the maintenance record, the production order and the quality event — three systems, three vocabularies, usually three different references for the same machine. That crossing is the work. It is also the reason a static dashboard never scales: every new question is a new dashboard.
A drop in OEE is a question — “why?” — whose answer usually spans production, maintenance and quality systems at once.
AutomAssist glossary
FAQ The questions that follow the formula.
- What is the OEE formula?
- OEE = Availability × Performance × Quality, all measured over the same window. Availability is run time ÷ planned production time; Performance is actual output ÷ theoretical output at nominal rate; Quality is good parts ÷ total parts.
- What is a good OEE score?
- 85% is the commonly cited world-class benchmark, from roughly 90% availability, 95% performance and 99% quality. It is a direction rather than a target: comparability depends entirely on how your planned production window is defined, so compare a line against its own history first.
- Is OEE the same as TRS?
- Yes. TRS — Taux de Rendement Synthétique — is the French term for OEE, and the two are calculated the same way. French practice adds two further indicators, TRG and TRE, defined alongside TRS in the AFNOR standard NF E60-182.
- Why does our OEE disagree between two systems?
- Almost always the window, not the maths. Different treatment of planned changeovers, breaks or micro-stops produces different denominators. Fix the convention before comparing numbers, and make each figure state the window it used.
- Do we need new sensors to improve OEE?
- Not to start. Availability moves first when diagnosis gets faster, and the evidence for that already sits in your CMMS, SCADA and MES records. Instrument the assets your own data shows deserve it, once you can see which those are.
Ask why the number moved.
Bring one line and last month’s OEE drop — we’ll show the maintenance, production and quality records answered together.
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