Glossary · Monitoring

MTBF (Mean Time Between Failures)

MTBF is the average operating time between one failure and the next for a repairable system, used with MTTR to estimate availability.

Updated October 5, 2026

MTBF is the average length of time a repairable system runs between failures. You calculate it by dividing total operating time by the number of failures in that period.

Why it matters

MTBF describes how often things break, while MTTR describes how long they take to fix. Together they give a steady-state availability estimate. A service that fails often but recovers in seconds can still be more available than one that fails rarely and takes hours to repair.

Example

Over a 30-day month (720 hours) a service had 3 failures totaling 90 minutes of downtime:

Operating time = 720 h - 1.5 h = 718.5 h
MTBF = 718.5 / 3 = 239.5 h
MTTR = 90 min / 3 = 30 min = 0.5 h
Availability = MTBF / (MTBF + MTTR) = 239.5 / 240 = 99.79%

Common mistakes

  • Treating MTBF as a prediction. With few failures, the number is mostly noise.
  • Using hardware spec-sheet MTBF for a whole service. Software bugs, deploys and dependencies dominate real outages.
  • Including planned maintenance as failures, or hiding failures that were "self-healed" by restarts.
  • Counting a flapping outage as one failure in some months and five in others. Define what a failure is first.

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