In brief
MTBF (mean time between failures) is the average operating time between two failures of a repairable system and a measure of its reliability.
Contents
What is MTBF?
MTBF is calculated as the total operating time divided by the number of failures in the period under review. If, for example, ten identical pumps each run for 5,000 hours and fail five times in total, the MTBF is 10,000 hours. The term assumes that the system is repaired after a failure and continues to run. For non-repairable components, such as many electronic parts, MTTF (mean time to failure) is used instead, the average time to the first failure.
MTTR (mean time to repair) describes the average time from the fault until the equipment is back in operation. Together, the two values give availability: MTBF divided by the sum of MTBF and MTTR. With an MTBF of 2,000 hours and an MTTR of 10 hours, availability is around 99.5 per cent.
Manufacturers' MTBF figures are not a statement of service life or a warranty. They apply to defined operating conditions and are often based on calculation models with a constant failure rate. Under this assumption, more than half of the units will have failed at least once by the time the MTBF is reached.
In practice: maintenance and procurement
Spare parts logistics mainly affects MTTR: time spent waiting for a part counts in full towards the repair time. For components with a low MTBF and long lead times, a spare part held in stock often improves availability more than any other measure.
Synonyms and abbreviations: mean time between failures · mean operating time between failures · MTTF · MTTR
Frequently asked questions
How do you calculate MTBF?
MTBF is the total operating time divided by the number of failures in that period. If several identical units are considered, their operating times are added together. Repair times do not count as operating time. The value only becomes meaningful with a sufficient number of failures and comparable operating conditions.
What is the difference between MTBF, MTTF and MTTR?
MTBF is the average operating time between two failures of a repairable system. MTTF describes the average time to failure of a non-repairable component, which is then replaced. MTTR is the average duration of a repair, including troubleshooting and recommissioning. Together, MTBF and MTTR determine availability.
Does an MTBF of 100,000 hours mean a device will last that long?
No. MTBF is a statistical average across many units, not a statement about the service life of an individual device. It only applies to the conditions defined by the manufacturer, such as temperature and load, and often excludes end-of-life wear. With a constant failure rate, a large proportion of devices fail well before the MTBF is reached.
How do you calculate availability from MTBF and MTTR?
Availability is MTBF divided by the sum of MTBF and MTTR. An example: with an MTBF of 500 hours and an MTTR of 5 hours, the result is 500 divided by 505, i.e. around 99 per cent. Shorter repair times, for example thanks to spare parts held in stock, increase availability just as fewer failures do.