MTBF Calculator
Mean time between failures for repairable items, from total operating time and the number of failures in the same period.
Formula
MTBF = Total operating time / Number of failures
What MTBF is and is not
MTBF applies to repairable items — equipment that is restored to service after a failure rather than discarded. For non-repairable items, the equivalent measure is mean time to failure.
Assumptions & boundaries
- Operating time should exclude idle and standby hours if your intent is to describe running behaviour.
- Only events your organisation classifies as failures should be counted; including minor stops or planned work changes the meaning.
- The result reflects the period measured. A short window with few failures carries large uncertainty.
- Aggregating dissimilar assets into one MTBF hides the behaviour of the worst performer.
Worked example
A compressor accumulated 4,200 operating hours in a year and suffered 6 failures.
MTBF = 4200 / 6 = 700.00 hours
The compressor ran, on average, 700 hours between failures. If MTTR for the same period was 14 hours, inherent availability would be 700 / (700 + 14) × 100 = 98.04%.
Frequently asked questions
- What does MTBF actually measure?
- MTBF is the average operating time accumulated between failures for a repairable item over the period you measured. It is an average of observed behaviour in that period, not a property fixed for the life of the asset.
- Is MTBF the expected life of the equipment?
- No. MTBF is not a guaranteed or expected life expectancy. An asset with an MTBF of 2,000 hours can fail after 40 hours or run for 6,000 hours. MTBF describes an average rate across a population or a period, not the age at which an individual item will fail.
- Should standby or idle time be included in operating time?
- Generally no, if your intent is to describe how long the item runs between failures. Including idle hours inflates MTBF. Whichever convention you adopt, apply it consistently and state it alongside the number.
- What if there were no failures in the period?
- MTBF is undefined with zero failures, because you would be dividing by zero. In that case report the failure-free operating time achieved, and note the observation window so the reader can judge how much evidence it represents.