How OEE is calculated
Overall Equipment Effectiveness multiplies three factors, each capturing a different category of loss:
OEE = Availability × Performance × Quality
Availability = Run Time ÷ Planned Production Time
Performance = (Ideal Cycle Time × Total Count) ÷ Run Time
Quality = Good Count ÷ Total Count
Availability = Run Time ÷ Planned Production Time
Performance = (Ideal Cycle Time × Total Count) ÷ Run Time
Quality = Good Count ÷ Total Count
Because the factors multiply, losses compound fast. A line running 90% on each factor — which sounds healthy — lands at just 72.9% OEE. That compounding is why OEE exposes problems that single metrics like utilization hide.
What is a good OEE score?
| OEE | Rating | What it means |
|---|---|---|
| 85%+ | World class | Benchmark for discrete manufacturing; sustained only with mature TPM |
| 60–85% | Typical | Where most measured plants sit; meaningful room to improve |
| 40–60% | Low | Common for lines new to OEE; large, fast gains available |
| <40% | Critical | Usually chronic unplanned downtime or severe speed loss |
Which factor to attack first
Look at whichever of the three factors is lowest — that is where the cheapest capacity is hiding:
- Low Availability — attack changeover time (SMED) and top breakdown causes. A downtime log with a simple Pareto chart usually shows 3–4 causes driving 80% of lost time.
- Low Performance — look for minor stops and lines deliberately run below rated speed. These losses rarely appear in any report because no one writes down a 40-second jam.
- Low Quality — focus on startup scrap after changeovers and drift within runs. First-pass yield by hour of shift usually points at the cause.
Common OEE mistakes
- Using rated speed instead of true ideal cycle time. If the ideal is set too slow, Performance can exceed 100% and the score is fiction.
- Excluding changeovers from downtime. Changeover is available time you chose to spend — hide it and you lose your biggest improvement lever.
- Averaging OEE across unlike machines. A plant-wide average hides which line is bleeding. Track OEE per constraint machine.