How to calculate OEE from one shift’s numbers

You calculate OEE by multiplying three factors: availability, performance and quality. Remove breaks and other time nobody scheduled to run from one shift to get planned production time, then divide run time by it, compare the parts made with what the ideal cycle time allows in that run time, and divide good parts by all parts. The product is the share of planned time that made good parts at full speed.

The example below follows one shift on a CNC lathe from the raw numbers to the result, then checks the answer a second way.

The OEE formula#

Each factor captures one kind of loss, and the OEE glossary entry lists how IoTFlows names each one. If you want the background first, What is OEE? covers what the number is for.

The formulas follow the standard OEE calculation.^[1] Multiply the factors; never add or average them. For example, a machine at 90% on all three factors has an OEE of 0.9 × 0.9 × 0.9 = 72.9%, not 90%.

Find planned production time first#

Every factor starts from this number, so settle it before you calculate anything.

Breaks come out of the shift because nobody intended to run the machine during them.^[1] A changeover stays in, because the machine was scheduled to produce. The six big losses, the standard breakdown of OEE losses, list setup and adjustments as an availability loss.^[2]

Take a stop out of planned production time only when nobody intended to run the machine during it. Keep it in when the machine was scheduled to run and didn’t, whatever the reason.

Planned and excluded are different things. A planned changeover is on the schedule, but it still costs production time, so it stays in. The glossary entry for planned downtime shows how to label those stops so you can report them apart from breakdowns.

Plants don’t all draw this line in the same place. The ISO 22400 standard itself gives two definitions of OEE, and a 2020 review found they seem to diverge from each other and from Seiichi Nakajima’s original formulation.^[3] Write your rules down once, then apply them to every machine and every shift, so this month’s number compares with last month’s.

Calculate each factor#

The example machine is a CNC lathe turning a steel shaft on an 8-hour day shift. The numbers are hypothetical and round, so you can recompute each step by hand.

Availability#

Run time is planned production time minus every stop inside it, changeovers included. Here, 480 − 30 = 450 minutes of planned production time, and 450 − 45 = 405 minutes of run time.

Availability is 405 ÷ 450 = 90.0%. The lathe was scheduled for 450 minutes and turned for 405 of them.

Performance#

Performance compares the ideal time for the parts you made with the time the machine actually ran.

The lathe made 360 parts at an ideal of 1.0 minute each, so they were worth 360 minutes at full speed. It ran for 405 minutes, so performance is 360 ÷ 405 = 88.9%.

The missing 45 minutes went to cycles slower than ideal and to short stops nobody recorded as downtime.

If one cycle makes several parts, divide the cycle time by the parts per cycle. For example, a 4-cavity mold with a 20-second cycle has an ideal of 5 seconds per part.

Performance should never exceed 100%. A reading above it means the ideal cycle time is set too slow.^[1] To set the number, time 10 to 20 consecutive cycles on a good day and use the best sustainable cycle, as the guide to setting the ideal cycle time describes.

Quality#

Quality is 342 ÷ 360 = 95.0%. The good count is the parts that passed the first time. A part that needs rework is not good, even if it ships after the rework.^[1]

The glossary entry for quality shows how good and bad counts appear on a shift report.

Multiply the factors#

Check the answer in lost minutes#

OEE has a short form that skips the three factors: good count × ideal cycle time ÷ planned production time.^[1] On the lathe, 342 × 1.0 ÷ 450 = 76.0%, the same answer. If the two forms disagree, one of the factors has an error.

The short form also says what OEE means in time. The lathe spent 342 of its 450 planned minutes making good parts at full speed. The other 108 minutes are losses, and each one belongs to exactly one factor.

Availability reads 90.0% and performance 88.9%, yet both lost 45 minutes. The percentages sit on different bases: availability on 450 minutes, performance on 405. Compare losses in minutes when you decide what to fix.

Mistakes that change the result#

The most common error divides performance by planned production time instead of run time. On the lathe, that gives 360 ÷ 450 = 80.0% and an OEE of 0.900 × 0.800 × 0.950 = 68.4%. The 45 stopped minutes already lowered availability, so dividing by 450 counts them a second time.

Each row below changes one thing on the same shift and leaves the rest as it was.

Which factor should you fix first?#

Start with the factor that loses the most minutes, not the one with the lowest percentage.

• Work on availability first when unplanned stops lead. Give every stop a reason and rank the reasons by total minutes.
• Work on performance first when the machine runs but output lags. Look for short stops nobody records and cycles slower than the ideal, and confirm the ideal cycle time is current.
• Work on quality first when scrap minutes lead, or when each reject wastes material worth more than the machine time it took.

You don’t need a combined OEE number to act on any of these. The factor with the most lost minutes tells you where to look. On the lathe, stops and slow cycles tie at 45 minutes each, so check which one is easier to cut this week.

Compare a machine with its own history before you compare it with a benchmark. Vorne notes that 85% is often called world class, and advises against comparing dissimilar processes with external benchmarks.^[4] Set the first goal a few points above the machine’s recent baseline, as the guide to choosing an OEE goal recommends.

Track the factors automatically#

A hand calculation covers one shift. Over a month, the inputs drift: short stops go unrecorded, counts get rounded and the ideal cycle time stays at whatever someone typed a year ago.

A SenseAi sensor on each machine records when it runs and stops, so availability arrives with no operator input. It counts parts against the ideal cycle time set on each operation, which gives performance. Quality comes from the good parts and the scrap your team logs with a reason.

IoTFlows reports OEE factor by factor, so each lost minute shows up under the factor that lost it. The metrics reference gives the definition behind each number.

Measured cycle times also test the numbers you plan with. Jesel, a valvetrain manufacturer, compares measured cycle times against the times in its bills of material, as its customer story describes.

Calculate OEE for your own shift#

1. Find planned production time

   Subtract breaks, meetings and periods with no work scheduled from the shift length.

2. Measure run time

   Subtract every stop inside planned production time, changeovers included.

3. Look up the ideal cycle time

   Use the best sustainable cycle for the operation, divided by the parts each cycle makes.

4. Count total and good parts

   Count every part off the machine, then the parts that passed inspection the first time.

5. Multiply the three factors

   Calculate availability, performance and quality, then multiply them.

6. Check with the short form

   Divide good count × ideal cycle time by planned production time, then list the lost minutes by factor.