MTTR Calculator
Mean time to repair from the total corrective repair time recorded and the number of repairs it covers.
Formula
MTTR = Total corrective repair time / Number of repairs
Boundaries: what counts as repair time
MTTR is unusually sensitive to definition. The same set of events can produce very different values depending on which elements your governance places inside the repair window:
- Diagnosis and fault finding — sometimes counted, sometimes tracked separately.
- Waiting time for spares, permits, isolation, crane or crew — often excluded from a strict maintainability definition and included in broader restoration measures.
- Functional testing and handover — included where return to service is the stop point, excluded where mechanical completion is.
Assumptions & boundaries
- Only corrective repairs should be included; planned maintenance durations belong elsewhere.
- Every event averaged must use the same start and stop points.
- Concurrent repairs on the same asset need a rule for how their time is counted.
- The mean alone does not describe the distribution; report spread or median where jobs vary widely.
Worked example
Six corrective repairs on a conveyor drive consumed 84 hours of repair time in total.
MTTR = 84 / 6 = 14.00 hours
If MTBF over the same period was 700 hours, inherent availability would be 700 / (700 + 14) × 100 = 98.04%.
Frequently asked questions
- Does MTTR include waiting for spares?
- That depends on your governance. A strict maintainability definition covers active repair work only. Many plants report a broader restoration time that includes diagnosis, waiting for parts or labour, and testing. Both are legitimate; mixing them in one trend is not.
- When does the repair clock start and stop?
- Define it explicitly. Common choices are from failure detection or from work order release, and stopping at handover to operations or at confirmed return to service. Changing the start or stop point shifts MTTR without anything changing in the workshop.
- Is a lower MTTR always better?
- Not automatically. Very short repair times can indicate temporary fixes that lead to repeat failures. Read MTTR alongside repeat-failure rate and MTBF before treating a reduction as an improvement.
- Should planned maintenance duration be included?
- No, if MTTR is meant to describe corrective repair. Planned job durations belong in schedule compliance and planning metrics. Combining them makes MTTR reflect workload mix rather than maintainability.