OEE (Overall Equipment Effectiveness) is the most-quoted maintenance indicator and, at the same time, the one most often calculated wrong. In the article on maintenance KPIs we covered the general formula; here we go step by step through how you get to the final number, with a full numeric example you can replicate on your own data.
The formula, in short
OEE = Availability × Performance × Quality
The three factors are multiplied together, not added or averaged: a problem in any one of them reduces the final result, even if the other two are perfect.
Step 1 — Availability
Availability measures how much time the machine actually produced compared to how much time it could have.
Availability = Actual run time / Planned production time
Planned production time is the shift minus the breaks that are already scheduled (lunch, shift change, planned maintenance), not the full 24 hours of the day. Actual run time is that figure minus unplanned stops (breakdowns, missing material, unexpected waits).
Example: an 8-hour, 480-minute shift with 30 minutes of scheduled breaks → planned production time of 450 minutes. During the shift, 45 minutes of unplanned stops are logged (one breakdown and one unscheduled changeover) → actual run time of 405 minutes.
Availability = 405 / 450 = 90%
Step 2 — Performance
Performance measures how much the machine produced compared to its rated speed, during the time it actually ran.
Performance = (Units produced × Ideal cycle time) / Actual run time
The ideal cycle time is the theoretical minimum time to produce one unit, stated by the machine’s manufacturer or measured under the best conditions — it is not the historical average cycle time, a common mistake covered further down.
Example: ideal cycle time of 0.5 minutes per unit (2 units per minute). Over the 405 minutes of actual run time, the theoretical maximum output would be 810 units. Actual units produced are 650, due to slowdowns and micro-stops not logged as full breakdowns.
Performance = (650 × 0.5) / 405 = 325 / 405 = 80%
Step 3 — Quality
Quality measures how much of the output produced is actually good, with no scrap or rework.
Quality = Good units / Total units produced
Example: of the 650 units produced, 20 are scrapped or need rework → 630 good units.
Quality = 630 / 650 = 96.9%
The final calculation
OEE = 90% × 80% × 96.9% = 69.8%
An OEE of 69.8% on this shift tells a different story than Availability alone, which looks good at 90%: in this case, the biggest room for improvement is in Performance, not in breakdowns.
Common calculation mistakes
- Using total time (24 hours) instead of planned production time when calculating Availability: it artificially inflates the denominator and hides the real impact of downtime.
- Confusing the ideal cycle time with the historical average cycle time. Using the historical average instead of the ideal value makes Performance come out close to 100% almost every time, even when the machine is genuinely slowing down.
- Calculating OEE over periods that are too long (a full month) instead of per shift or per day: it hides the variation and makes it impossible to tell which shift or which specific cause is pulling the number down.
- Not breaking the three factors apart when reporting the result. A low OEE, without knowing whether it comes from Availability, Performance or Quality, doesn’t tell anyone what to do next.
What to do with the number
OEE taken on its own, once, is not very useful. Its real value shows up when it’s tracked over time, per machine, and broken down into its three factors: only then does it become clear whether the room for improvement is in maintenance (Availability), in the process (Performance) or in quality control (Quality) — and therefore who should own the next action.