How to calculate it
Divide the time the equipment has been in service by the number of failures in that period. A machine that has run for 1,800 hours and failed 3 times has an MTBF of 600 hours. The higher it is, the more reliable the equipment.
What it is not
It is not the service life, nor a promise that the equipment will last that long. It is an average: it helps you compare equipment and spot trends, not predict when a specific machine will fail.
What it is for
- Comparing two machines of the same family and spotting the one that is deteriorating.
- Justifying a replacement with data rather than gut feeling.
- Sizing spare parts stock to the real failure rate.
- Knowing whether the preventive plan works: if MTBF goes up, it works.
The usual mistake
Calculating it on an incomplete history. If half the breakdowns were fixed without logging a work order, MTBF comes out twice as good as it really is. The metric is only as good as the records behind it.