Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a system, during normal system operation.
MTBF can be calculated as the arithmetic mean (average) time between failures of a system. The term is used for repairable systems, while mean time to failure (MTTF) denotes the expected time to failure for a non-repairable system.
By referring to the figure above, the MTBF of a component is the sum of the lengths of the operational periods divided by the number of observed failures:
MTBF is an important marker in reliability engineering and has its roots in the aviation industry, where airplane failure can result in fatalities.
Although MTBF does not consider planned maintenance, it can still be applied for things like calculating the frequency of inspections for preventive replacements.