Maintenance teams are often judged by how busy they look. MTTR and MTBF change that conversation: they measure how quickly the team restores machines and how reliably machines run between failures. Both are simple to calculate once breakdowns are recorded properly, and both point directly to improvement opportunities.
What is MTTR?
MTTR — Mean Time To Repair (sometimes Mean Time To Restore) — is the average time taken to bring a failed machine back into service.
MTTR = total repair time ÷ number of repairs
Repair time normally runs from when the failure is reported to when the machine is available for production again. Some organisations measure only active repair time (excluding waiting for a technician or spares). Choose one definition, write it down and apply it consistently — the trend matters more than the absolute value.
What is MTBF?
MTBF — Mean Time Between Failures — is the average operating time between one failure and the next.
MTBF = total operating time ÷ number of failures
Operating time is the time the machine was meant to be running — not calendar time. A machine running one shift a day has much less operating time than one running three shifts, and MTBF should reflect that.
A worked example
A CNC machine runs two shifts of 8 hours, 25 days a month: 400 planned operating hours. In the month it has 4 breakdowns, with repair times of 2, 1.5, 4 and 0.5 hours — 8 hours in total.
- MTTR = 8 hours ÷ 4 repairs = 2 hours.
- Operating time excluding breakdowns = 400 − 8 = 392 hours.
- MTBF = 392 ÷ 4 = 98 hours.
So the machine fails roughly every 98 running hours, and each failure takes about 2 hours to fix.
How MTTR and MTBF relate to availability
Inherent availability can be estimated as MTBF ÷ (MTBF + MTTR). In the example: 98 ÷ (98 + 2) = 98%. To improve availability you can either fail less often (raise MTBF) or recover faster (reduce MTTR). Both feed into the availability component of OEE.
What counts as a failure?
Agree on a definition. Typically, a failure is any unplanned stop requiring maintenance intervention. Very short stops cleared by operators (a jam, a reset) are usually tracked as minor stoppages for OEE rather than failures for MTBF. Planned maintenance, changeovers and material shortages are not failures. Clear rules prevent arguments and keep metrics comparable between machines and months.
Breaking MTTR down
MTTR is more useful when split into components:
- Response time — from report to technician arrival.
- Diagnosis time — finding the cause.
- Waiting for spares — often the largest and most fixable element.
- Repair time — the actual fix.
- Testing and handover — confirming the machine is ready.
If waiting for spares dominates, stock critical spares. If diagnosis dominates, improve documentation, training or troubleshooting guides. If response dominates, review how breakdowns are reported and dispatched.
Improving MTBF
MTBF improves when failures are prevented: effective preventive maintenance, operator care (cleaning, lubrication, inspection), fixing root causes rather than symptoms, and replacing components before they reach end of life. Breakdown history by failure mode shows which actions will have most effect. See preventive maintenance software.
Common calculation mistakes
- Using calendar time instead of operating time for MTBF.
- Mixing planned and unplanned downtime.
- Calculating MTBF on very few failures and treating it as precise.
- Averaging MTTR across very different machines.
- Inconsistent start and end times for repairs.
MTTF, MTTA and related terms
MTTF (mean time to failure) is used for items that are replaced rather than repaired, such as bearings or lamps. MTTA (mean time to acknowledge) measures how quickly a breakdown is acknowledged. They follow the same logic and can be useful for specific components or response processes.
How software calculates them
A maintenance system records each breakdown with machine, reported time, start of repair, end of repair, cause and spares used. With planned operating time from the production calendar, MTTR and MTBF are calculated automatically per machine, line and period — and trended. Technicians record breakdowns on their phones; managers see the metrics without spreadsheets. See CMMS software.
Using the metrics in practice
Review monthly: which machines have the lowest MTBF, which have the highest MTTR, and what the top failure causes are. Assign actions to the top few. Check in the following months whether the numbers moved. That simple routine turns metrics into reliability improvement.
Corposol builds maintenance software that calculates MTTR, MTBF and downtime automatically. Talk to us.
Presenting the metrics to management
Numbers alone rarely change decisions. Present MTTR and MTBF alongside their business impact: hours of production lost, orders delayed and the cost of emergency repairs. Show the trend over six or twelve months and the actions taken. When management sees that preventive maintenance time protects output, it becomes far easier to secure the planned stops and spares budget that reliability needs.
Frequently asked questions
What is a good MTTR?
There's no universal number; it depends on equipment complexity. Track your own MTTR per machine over time and aim to reduce it through better spares availability, diagnostics and procedures.
Should planned maintenance count as a failure?
No. MTBF counts unplanned failures. Planned maintenance is scheduled downtime and is tracked separately.
Can MTBF be calculated for a machine with no failures?
Not meaningfully. If a machine has no failures in the period, report operating time and zero failures rather than an MTBF figure.
Do we need sensors to calculate these metrics?
No. Breakdown start and end times recorded in a maintenance app are enough; machine signals simply make timing more precise.
How often should we review MTTR and MTBF?
Monthly for trends, with a quarterly deeper review of critical machines.
Should MTTR include time waiting for spare parts?
If you measure time to restore, yes — waiting for spares is part of what the production team experiences. Recording it as a separate component of repair time shows how much of MTTR could be removed by stocking critical spares.
