Availability is the proportion of time a system is in a functioning condition. This is often described as a mission capable rate. Mathematically, this is expressed as 1 minus unavailability. Availability combines MTBF with a second metric called mean time to repair (MTTR) that measures the time required to acknowledge a problem, respond to it and complete a repair. Availability is typically expressed as a number of “nines” representing the percentage of time over a year’s worth of use that a given system is operational. For example, a device with an MTBF of 500,000 hours and a MTTR of four hours would have an availability of .999992, or 99.9992 percent (500,000 ÷ 500,004). That translates to an expected downtime of 4.2 minutes per year.
