Key takeaways
Short answer: MTTR (mean time to repair) is how long it takes to get a failed asset running again. MTTF (mean time to failure) is the expected lifetime of a non-repairable item — a bearing or a bulb you replace rather than fix. MTBF (mean time between failures) is the cycle between failures of a repairable asset. They answer different questions, and mixing them up corrupts availability calculations and maintenance strategy alike. See also mtbf vs availability.
Mean time to repair measures maintainability — the average time from failure to restored operation. It includes diagnosis, parts, the repair itself and verification. Long MTTR is often driven less by the wrench time than by waiting: waiting for a diagnosis, waiting for a long-lead spare, waiting for a technician. It is the lever maintenance speed pulls.
Mean time to failure applies to items you do not repair — you run them until they fail and then replace them. A sealed bearing, a filter, a light bulb. MTTF describes how long, on average, that item lasts, and it drives replacement planning and run-to-failure decisions for non-repairable components.
Mean time between failures applies to repairable assets — a pump, a motor, a machine — that fail, get fixed, and run again. It is the average run time between those failures, a pure reliability measure improved by removing failure causes. The distinction from MTTF matters: MTBF assumes repair and reuse; MTTF assumes replacement.
A pump (repairable) has an MTBF of 1,200 hours and an MTTR of 8 hours, so its availability is 1200 / (1200 + 8) = 99.3%. Inside that pump sits a seal (non-repairable) with an MTTF of 1,200 hours — it does not get repaired, it gets replaced at each failure. Reporting the seal's life as "MTBF" or the pump's repair time as "MTTF" would scramble the availability maths and the replacement plan. Each metric describes a different thing about the same asset, and only used correctly do they combine into a meaningful availability number.
Availability equals MTBF divided by (MTBF plus MTTR) — reliability over reliability plus recovery. MTTF feeds the replacement and run-to-failure decisions for the throwaway components inside that asset. Get the metric wrong and the availability figure, the spares plan and the maintenance strategy all inherit the error.
1. Calling a non-repairable item's life MTBF. It is MTTF — there is no "between failures" for a part you replace.
2. Optimising MTBF while ignoring MTTR. Slow repairs drag availability down even on reliable assets.
3. Treating MTTR as wrench time only. Spare lead time and diagnosis are usually the bigger drivers.
4. Comparing the three as if interchangeable. They measure different things and do not substitute.
Availability — one of OEE's three terms — is built from MTBF and MTTR. Reliability engineering raises MTBF, faster response and better spares planning cut MTTR, and MTTF guides which throwaway parts to stock. Read together, the three tell you exactly which lever is limiting uptime.
See how Fabrico captures this automatically on your lines — explore OEE for manufacturing or book a demo.
MTBF is for repairable assets (fail, fix, reuse); MTTF is for non-repairable items you replace at failure.
Yes — MTTR covers diagnosis, parts, repair and verification, and spare lead time is often the biggest part.
Availability equals MTBF divided by (MTBF plus MTTR).
It guides replacement and run-to-failure decisions for non-repairable components.
Yes — MTBF and MTTR for the repairable asset, MTTF for the throwaway parts inside it.
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