The mean time expected to the first failure of a piece of equipment. It 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, but MTBF is commonly used for both repairable and non-repairable items.