Daren Matthews

Mean Time Between Failures (MTBF)

MTBF is a basic measure of reliability for components that are able to be repaired. It is the number of hours that pass before a component or system fails. It is a commonly-used variable in network availability analyses and reports.

MTBF may be calculated as the inverse of the failure rate for constant failure rate systems. For example: If a component has a failure rate of 2 failures per million hours, the MTBF would be the inverse of that failure rate.

MTBF = (1,000,000 hours) / (2 failures) = 500,000 hours

Mean Time To Failure (MTTF)

MTTF is a basic measurement of reliability for systems which are non-repairable (such as the battery in a UPS). It is the mean time expected until the first failure of a piece of equipment. MTTF is a statistical value and is meant to be the mean over a long period of time and large number of units.

For constant failure rate systems, MTTF is the inverse of the failure rate. If failure rate is in failures/million hours, MTTF = 1,000,000 / Failure Rate for components with exponential distributions.

Technically MTBF should be used only in reference to repairable items, while MTTF should be used for non-repairable items. However, MTBF is commonly used for both repairable and non-repairable items.