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MTBF, MTTR and availability, explained
Three numbers describe how reliable a machine has been: how long it runs between failures, how long a repair takes, and what share of the time it was available. They are only as good as the stoppage records behind them.
The definitions
- MTBF, mean time between failures: operating time divided by the number of failures.
- MTTR, mean time to repair: total repair time divided by the number of repairs.
- Availability: uptime divided by total time, or equivalently MTBF divided by MTBF plus MTTR.
A worked example (illustrative numbers)
A machine is scheduled to run 720 hours in a month and stops unplanned 4 times, with 12 hours of total repair time. Operating time is 708 hours, so MTBF is 177 hours. MTTR is 12 divided by 4, which is 3 hours. Availability is 708 divided by 720, about 98.3 percent. These numbers are an example only.
What the numbers tell you
A low MTBF says the machine fails often: look at root causes and preventive work. A high MTTR says repairs take long: look at spare parts, access, documentation and skills. Availability combines both, but two machines with the same availability can have very different problems.
Mistakes that make the numbers misleading
- Small samples. Two or three events say little. Treat the result as a hint, not a verdict.
- Inconsistent definitions. Decide what counts as a failure and whether planned stops are excluded, and keep it the same.
- Missing events. If short stoppages are not logged, MTBF looks better than reality.
- Wrong end times. A stoppage left open, or closed late, distorts MTTR.
- Blending different machines. Calculate per machine, not across a whole line.
How Plant Cortex handles reliability numbers
Plant Cortex works these figures out from the stoppages people already log, per machine, over a set window. A machine with fewer than three events is marked low confidence rather than hidden. Cost figures appear only if someone has entered an hourly rate, and are never estimated.
General information, not legal advice. Check current regulations and your own site procedures.
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