Availability is the probability that an item is in an operable state at a specified point in time. Like reliability, availability is a probability value, with values ranging from 0 to 1. Unlike reliability, availability takes into account repair times. The equation for the calculation of steady state availability is:

Availability = MTBF / MTBF + MTTR

Assume that the MTBF of a repairable item is 50 hours. This means that the item can be expected to operate for approximately 50 hours without failure. When the item fails, the time needed to repair it (MTTR) and put it back into operation is 10 hours. Items that have high MTBF values and low MTTR values have high availability.

As MTBF decreases and/or MTTR increases, availability decreases. This means that at a certain time, the system is more likely to be down, or in a failed state. Availability is especially important for critical systems. Availability is also important when analyzing various modeling options and the implementation of redundant components.