Reliability and uptime
MTBF calculator
Mean Time Between Failures is the average operating time between one unplanned failure and the next. It is a measure of reliability, and it applies to repairable assets: equipment you fix and put back into service.
MTBF = Total operating hours ÷ Number of failures
Hours the asset was actually running over the period
Unplanned failures only
Show the assumptions
- Formula
- MTBF = Total operating hours ÷ Number of failures
Operating hours, not calendar hours. Time the asset sat idle but healthy is not time it was proving its reliability.
MTBF is an average, so it says nothing about the spread. An asset that fails every 600 hours like clockwork and one that runs 4,000 hours then fails six times in a fortnight can report the same MTBF and need completely different interventions.
MTBF
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Prefilled with an example. Change any number and the result updates.
How to calculate MTBF
MTBF = Total operating hours ÷ Number of failures
Operating hours, not calendar hours. Time the asset sat idle but healthy is not time it was proving its reliability.
Worked example
- Total operating hours 4,200 hrs
- Number of failures 7
MTBF = 600.0 hrs
Reading the result
MTBF is an average, so it says nothing about the spread. An asset that fails every 600 hours like clockwork and one that runs 4,000 hours then fails six times in a fortnight can report the same MTBF and need completely different interventions.
MTBF questions
What is the difference between MTBF and MTTF?
MTBF is for repairable assets and measures the operating time between failures. MTTF is for items you replace rather than repair, such as a bearing, a lamp or a sensor. It measures the operating time until the single failure that ends its life.
Should planned maintenance downtime be excluded from MTBF?
Yes. MTBF counts operating hours against unplanned failures. Including planned downtime in the numerator inflates the figure, and counting a planned intervention as a failure deflates it. Both mistakes are common and they do not cancel out.
How many failures do I need before MTBF means anything?
More than a couple. With one or two failures the figure moves wildly with each new event, and any decision it supports is really a decision based on one data point. Quote the failure count alongside the MTBF so whoever reads it can judge for themselves.
Related calculators
Track failures by asset
UpKeep records the work, the hours and the failures this metric depends on, so MTBF is a report rather than a spreadsheet exercise.