Circuit reliability (also time availability) (CiR) is the percentage of time an electronic circuit was available for use in a specified period of scheduled availability. Circuit reliability is given by
- <math>T_s = T_a + T_o</math>
where <math>T_o</math> is the circuit total outage time, <math>T_s</math> is the circuit total scheduled time, and <math>T_a</math> is the circuit total available time. In addition, circuit reliability is the expected lifespan of operation of a functioning system under nominal conditions.