What Is MTTF? Mean Time to Failure Explained

MTTF (mean time to failure) measures the average time a system runs before it fails. It sounds like MTTR, but measures the opposite: MTTR is how quickly you recover after a failure, MTTF is how long the system runs before the next one. A server that goes down once every 90 days has an MTTF of 90 days. A high MTTF means incidents are rare — but says nothing about how fast you get out of them.

Historically, MTTF applied to components that aren't repaired — hard drives, power supplies, light bulbs. A manufacturer quotes an MTTF of 100,000 hours, meaning that's the average lifespan before the unit dies. In software systems the term is used more loosely: it describes the average interval between incidents, even when the system is restored every time.

MTTF vs MTTR — why you need both

MTTF tells you how often something breaks. MTTR tells you how fast it gets fixed. A service with an MTTF of 30 days and an MTTR of 5 minutes can feel more reliable to users than one with an MTTF of 90 days and an MTTR of 2 hours — it breaks more often, but users barely notice. That's why SLAs and SLOs work in uptime percentages rather than failure frequency alone: what matters is the combination of both metrics.

MTTF and monitoring

Monitoring can't directly increase MTTF — that's a question of architecture, code quality, and infrastructure. But uptime monitoring provides the data to calculate it: months of incident history show the average interval between failures. And SSL and domain monitoring prevent predictable failures like expired certificates — effectively raising MTTF by eliminating an entire class of incidents.

Related terms: MTTR, uptime, SLA, SLO