MTBF (mean time between failures) is the average time between failures. The units for MTBF are always hours. MTBF is used to evaluate repairable items. It is assumed that a repairable item can experience multiple failures in its lifetime. Upon failure, a repair restores the item to operation. MTBF can be described as the number of hours that are expected to pass before a failure occurs.

For constant failure rate systems, MTBF is the inverse of the failure rate (FR) for the repairable item: MTBF = 1,000,000 / FR

For example, if a part fails twice in a million-hour time period, it has a failure rate of 2 failures per million hours: 1,000,000 / 2 = 500,000 hours

The MTBF is the inverse of that failure rate: 1,000,000 / 2 = 500,000 hours

An MTBF of 500,000 indicates that the part can be expected to fail approximately every 500,000 hours. It is important to understand that MTBF is not a definite value. If a part has an MTBF of 8,000 hours, it means that on average that part fails every 8,000 hours. It does not mean that the part will necessarily continue to operate for 8,000 hours or will cease to operate after 8,000 hours.