Formulas
MTBF = Total operating time / Number of failures
MTTR = Total corrective repair time / Number of repairs
Combined, they give the inherent form of availability:
Inherent Availability = MTBF / (MTBF + MTTR) × 100
Definitions
MTBF
Mean operating time accumulated between failures, for a repairable item. It is an average over the period measured, not a guaranteed life. For items that are discarded rather than repaired, the corresponding measure is mean time to failure.
MTTR
Mean time to repair, or restore, per corrective event. Its value moves sharply with the boundary you set: whether diagnosis, waiting for spares, permitting and functional testing sit inside or outside the clock is a governance decision that must be applied consistently.
Worked example
Two identical pumps are each measured over 4,200 operating hours.
- Pump A: 6 failures, 84 hours total repair time.
- Pump B: 2 failures, 84 hours total repair time.
Pump A: MTBF = 4200 / 6 = 700 h · MTTR = 84 / 6 = 14 h
Pump B: MTBF = 4200 / 2 = 2100 h · MTTR = 84 / 2 = 42 h
Both lost the same 84 hours, so both show similar inherent availability — 98.04% for A and 98.04% for B. But they need different work. Pump A fails often and is restored quickly: look at failure modes, operating context and PM content. Pump B rarely fails but takes a long time to restore: look at spares holding, accessibility, procedures and response coordination.
Common mistakes
- Treating MTBF as expected life. An MTBF of 2,100 hours does not promise 2,100 failure-free hours for any individual pump.
- Comparing MTTR values built on different clocks. One site’s “active repair only” and another’s “detection to return to service” are not the same quantity.
- Including idle hours in MTBF operating time. This inflates MTBF without any change in equipment behaviour.
- Celebrating a falling MTTR alone. Faster restoration achieved through temporary fixes often shows up later as a falling MTBF.
- Averaging dissimilar assets. A fleet MTBF can look acceptable while one bad actor generates most of the events.