How to calculate OEE, and why most OEE numbers are wrong
The formula is simple. What is usually wrong is the denominator and the hand-written downtime log.
OEE is availability multiplied by performance multiplied by quality. Availability is how much of planned production time the machine ran. Performance is how close it came to theoretical speed while running. Quality is the share of output that was good.
Three things that make OEE numbers wrong
Shrinking the denominator. Removing downtime from planned production time inflates availability. Calling every stop unplanned turns OEE into a meaningless number. The rule: the plan is whatever it was before the stop happened.
Logging downtime by hand. Times get rounded and short stops are never recorded — yet short stops are where most of the loss lives. The system should count the minutes; the operator should only pick the reason.
An unrealistic theoretical cycle time. The standard should be the best rate actually achieved, not the catalogue figure.