Availability

The capacity of a system to perform its intended function when called upon to do so is often referred to as availability of operational readiness. In the simplest two state case, where equipment or system is either working at full capacity or not working the availability is commonly defined as the probability of the system operating satisfactory and is represented as

Availability = Total Up Time / (Total Up Time + Down Time)

Mathematically, it can be expressed as the ratio of mean time between failures (i.e. operating time) to the total available time of the system. Thus,

Availability = MTBF / (MTBF + MTTR)