What is MTBF?
MTBF (Mean Time Between Failures) is the average time between successive failures of a system. It measures how frequently a system fails and is one of the core reliability metrics alongside MTTR (Mean Time to Recovery) and MTTF (Mean Time to Failure).
MTBF = MTTF + MTTR
Or equivalently: MTBF = Total observation period ÷ Number of failures
MTBF vs MTTF vs MTTR
| Metric | Measures | Formula | Goal |
|---|---|---|---|
| MTBF | Time between failures | MTTF + MTTR | Higher is better |
| MTTF | Time until next failure | Uptime ÷ Failures | Higher is better |
| MTTR | Time to recover from failure | Downtime ÷ Failures | Lower is better |
MTBF and MTTF are often confused. The difference: MTTF covers only the uptime between failures. MTBF covers the full cycle — uptime plus recovery time. For systems that are repaired and returned to service, MTBF is the more useful metric.
MTBF and availability
MTBF directly determines availability: Availability = MTTF ÷ (MTTF + MTTR) = MTTF ÷ MTBF
This relationship shows that availability can be improved in two ways: increasing MTTF (failing less often) or decreasing MTTR (recovering faster). In practice, reducing MTTR is often faster and cheaper than preventing failures entirely.
Failure rate
Failure rate is the inverse of MTTF: Failure rate (λ) = 1 ÷ MTTF. It expresses how many failures to expect per unit of time. A system with MTTF of 100 hours has a failure rate of 0.01 failures per hour, or roughly 87.6 failures per year.
Failure rate is most useful when comparing systems or components — a lower failure rate means more reliable hardware or software.
When to use MTBF
MTBF is most meaningful for repairable systems where the same service is brought back online after each failure. For one-time components (light bulbs, hardware that is replaced rather than repaired), MTTF is the appropriate metric. For software services, MTBF is almost always the right choice since services are restored rather than replaced.
Use MTBF alongside the MTTR Calculator to get a complete picture of your service's reliability profile, and the Availability Calculator to see how your MTBF translates to uptime percentage.