OEE: what it is and how it is calculated

In one line

OEE (Overall Equipment Effectiveness) answers a single question: of all the time you planned to produce, what percentage ended up as good production, at the expected pace?

The formula: three factors, not one

OEE = Availability × Performance × Quality

• Availability: the time the equipment actually operated, over the time it was scheduled to operate. It drops because of unplanned stoppages (failures) and planned stoppages that are recorded here (product changeovers, adjustments).
• Performance: how close the actual production speed was to the ideal/nominal speed of the equipment. It drops because of micro-stoppages and running slower than normal, even if the equipment never stopped completely.
• Quality: what percentage of the parts produced met specification on the first attempt. It drops because of scrap and parts that need rework.

Vorne illustrates this with a classic example: an 8-hour shift (480 min), with 60 min of planned stoppages, leaves 420 min available. If the equipment was stopped 47 more minutes because of failures, Availability is (420-47)/420 ≈ 89%. Multiplying the three factors this way, an OEE of 85% is considered world class for discrete manufacturing. Most plants, without systematic measurement, are well below that.

The Vorne guide also gives the short form: OEE = (good parts × ideal cycle time) ÷ planned production time, and the three formulas with the data of one shift: Availability = operating time ÷ planned time; Performance = (total parts ÷ operating time) ÷ ideal speed; Quality = good parts ÷ total parts. The guide recommends including breaks and product changeovers within the planned production time, so that all losses are recorded and the real capacity of the equipment is exposed, and measuring OEE at the bottleneck of the process. Watch out for one difference: Vorne's example spreadsheet subtracts 60 minutes of breaks from the 480 minutes of the shift to arrive at 420 planned. What matters is to set a criterion and stick to it so you can compare against yourself.

Calculate the OEE of your shift

Preloaded is Vorne's worked example (an 8-hour shift with 60 minutes of breaks, 47 minutes of stoppage, 60 parts per minute, 19,271 parts and 423 rejects), which gives 74.79%. Besides the OEE, the calculator shows you how many minutes of the shift are lost in each factor. You can also track it shift by shift with the OEE Excel spreadsheet.

The Six Big Losses

Each of the three OEE factors has two associated loss categories. They are the map for knowing where to attack when the number is low:

Availability losses

1. Equipment failures (unplanned breakdowns)
2. Setup and adjustments (product changeovers, shift start-up). Here it connects directly with SMED: reducing die changeover time raises this factor without touching anything else

Performance losses 3. Micro-stoppages (the equipment stops for seconds or minutes, clears itself, and often is not even recorded) 4. Reduced speed (it runs slower than its nominal speed, because of wear, poor adjustment, or conservative operation)

Quality losses 5. Production rejects (parts out of specification from the start) 6. Start-up rejects (defective parts specifically during start-up or process warm-up)

What it is for in practice

OEE is not a number to report and file away: it is used to decide where to focus improvement. If the factor that hurts a line's OEE the most is Performance (micro-stoppages), investing in reducing major failures (Availability) will move the needle very little. The focus should be on the short, frequent stoppages, which usually go unnoticed because nobody records them one by one.

Limitations to keep in mind

• OEE compares against the "ideal" speed of the equipment. If that reference value is poorly defined (too optimistic or out of date), the result loses its meaning
• It is useful for comparing the same equipment against itself over time; comparing the OEE of very different pieces of equipment says little
• A high OEE on equipment that is not the line's bottleneck does not improve total output. You have to look at OEE together with where the real constraint of the process is

In summary

OEE reduces three different questions (did it operate? at what speed? with what quality?) to a single number, and the Six Big Losses explain where the gap against 100% comes from. The real value is not in the final number but in identifying which of the three factors is pushing it down the most.