Maintenance and reliability
What is MTBF (mean time between failures)?
MTBF is the average operating time between failures of a repairable machine. Formula, worked example, what MTBF does not tell you, and a free MTBF calculator.
Quick answer
MTBF (mean time between failures) is the average operating time a repairable machine runs between one failure and the next. It equals total operating time divided by the number of failures in the period. Higher MTBF means a more reliable machine. It is used with MTTR to calculate availability.
Updated 30 October 2026. Written by the Ajinkya Technologies engineering team, reviewed by Amey Kadle.
Formula
MTBF = total operating time ÷ number of failures
Worked example: A compressor ran 720 hours in a month and failed 4 times. MTBF = 720 ÷ 4 = 180 hours. Its failure rate is 4 ÷ 720 = 5.6 failures per 1,000 operating hours.
MTBF calculator
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- MTBF
- 180 hours
- Failure rate
- 5.56 failures per 1,000 hours
MTBF (mean time between failures) explained
MTBF applies to repairable equipment. For items that are replaced rather than repaired, such as a bearing or a fuse, the equivalent measure is MTTF, mean time to failure. Both use operating time, not calendar time, so a machine that runs one shift a day is not penalised for the hours it is switched off.
MTBF is an average and says nothing about when the next failure will happen. A machine with an MTBF of 180 hours may fail twice in a day and then run for weeks. It is most useful as a trend: rising MTBF shows that reliability work is paying off, falling MTBF flags a machine or failure mode that needs attention. Comparing MTBF between machines only works if failures are defined the same way, for example every stop that needed a maintenance intervention.
The quality of MTBF depends on the quality of the failure records. When breakdowns are logged on paper or in a spreadsheet, short failures and repeat faults are often missed. Capturing stops automatically from the machine and linking each one to a maintenance work order gives reliable MTBF per asset and per failure mode, which is the input predictive maintenance needs.
Key concepts
- For repairable equipment; MTTF for replaceable items
- Uses operating time, not calendar time
- An average, not a prediction of the next failure
- Most useful as a trend per asset
- Needs consistent failure definitions
Questions
What is the difference between MTBF and MTTF?
MTBF is used for repairable equipment and measures average time between failures. MTTF is used for non repairable items and measures average time until failure.
What is a good MTBF?
There is no universal value; it depends on the equipment type, duty and how failures are defined. Track MTBF per asset over time and compare like with like.
How do MTBF and MTTR give availability?
Inherent availability = MTBF ÷ (MTBF + MTTR). With an MTBF of 180 hours and an MTTR of 2.5 hours, availability is 98.6%.
Does MTBF tell me when to do maintenance?
Not on its own. It is an average. Condition monitoring and predictive maintenance use live machine data to estimate when a specific failure is developing.
References
How we help with MTBF
Our maintenance and MES systems log every stop from the machine and link it to a work order, so MTBF and MTTR per asset are calculated automatically and trend over time.
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