MTBF (Mean Time Between Failures) and MTTR (Mean Time To Repair) are two core metrics that make the reliability and availability of installations measurable for maintenance and asset teams. MTBF describes how long a repairable asset runs failure-free on average; MTTR describes how long it takes on average to resolve a failure. Together they tell the full story: how often something breaks, and how quickly it is running again.
What is MTBF?
MTBF is the average operating time between two consecutive failures of a repairable asset, such as a pump, conveyor or compressor. It is a measure of reliability: the higher the MTBF, the longer an asset functions without interruption on average. Time is counted while the asset is in operation; downtime for planned maintenance is usually not counted as failure time.
What is MTTR?
MTTR is the average repair time: the time between a failure occurring and the moment the asset is productive again. It often includes diagnosis, waiting for parts, the actual repair and restarting. MTTR is a measure of how effective your maintenance process is: the lower the MTTR, the faster you recover.
The difference with MTTF
Alongside MTBF there is MTTF (Mean Time To Failure). This metric applies to non-repairable items that are replaced rather than repaired after a failure, such as a bearing, lamp or fuse. Where MTBF measures the time between failures of the same asset, MTTF measures the expected lifetime until the first (and only) failure. In practice you use MTBF for repairable systems and MTTF for disposable or replaceable components.
How do you calculate MTBF and MTTR?
The formulas are simple:
- MTBF = total operating time / number of failures
- MTTR = total repair time / number of repairs
A short example. Suppose a production line runs for 600 hours in a month and fails 4 times in that period. The MTBF is then 600 / 4 = 150 hours: on average the line runs 150 hours between failures. If repairing those 4 failures took 20 hours in total, the MTTR is 20 / 4 = 5 hours per failure.
MTBF, MTTR and availability
The real value emerges when you combine both figures. The (inherent) availability follows from:
- Availability = MTBF / (MTBF + MTTR)
Using the numbers from the example: 150 / (150 + 5) = 150 / 155 ≈ 0.968, or roughly 96.8% availability. The formula shows immediately what you can steer. Raising MTBF makes the asset more reliable — there are simply fewer failures. Lowering MTTR speeds up recovery — each failure costs less production time. Both movements push availability up, but through a different lever: breaking down less often, or getting back up faster.
This also makes the two metrics diagnostic. Low availability with a high MTBF but high MTTR points to a slow recovery process (parts, knowledge, procedures). A low MTBF points instead to an underlying reliability problem that you need to address structurally, for example through reliability and FMECA.
Pitfalls: averages and registration
Both figures are averages, and averages hide variation. An MTTR of 5 hours could consist of four repairs of 5 hours each, or of three quick jobs of half an hour and one outlier of 18.5 hours. For your planning and risks that makes a world of difference. So look beyond the average at the spread and at outliers.
The biggest pitfall, however, is garbage in, garbage out. MTBF and MTTR are no more reliable than the data they rest on. Missing failure reports, inaccurate start and end times or inconsistent definitions of "failure" make the figures worthless or, worse, misleading. Scope matters too: count only operating time, define clearly what qualifies as a failure, and stay consistent across assets and periods.
The link with maintenance
Reliability metrics are a direct product of good maintenance records. Every failure and every repair must be logged as a work order, with correct timestamps and a cause code. That registration is the foundation on which MTBF and MTTR rest. A system that structurally tracks work orders and failures — see what a CMMS is — supplies the source data, and clear reports and dashboards make the trends visible. This is how you shift from firefighting to steering on facts: where is MTBF declining, and where is MTTR too high?
Frequently asked questions
What is a good MTBF or MTTR value?
There is no universal standard; it depends on the type of asset, the business context and the availability requirements. More meaningful than an absolute target is the trend: if your MTBF rises and your MTTR falls over time, your reliability and recovery capability are improving. Compare above all with your own history and with similar assets.
What is the difference between MTBF and MTTF?
MTBF applies to repairable assets and measures the average operating time between failures. MTTF applies to non-repairable items that you replace after failure, and measures the expected lifetime until the single failure. In short: MTBF is about repairing, MTTF is about replacing.
How do I improve my availability?
Through two levers. Raise MTBF with preventive and condition-based maintenance and with analyses such as FMECA to address recurring failure modes. Lower MTTR with faster diagnosis, available spare parts and clear repair procedures. Together they raise availability the fastest.