1. One or more non-transitory computer readable storage media storing code for calculating an availability value for a system having a plurality of operational units and unit combinations that, when executed by one or more processors configures the processors to: receive, from a user interface, one or more operational reliability metrics for each operational unit in an operational state, the operational unit being selected from the plurality of operational units of the system; designate that one or more spare reliability metrics for at least one spare unit associated with the operational unit are similar to the one or more operational reliability metrics such that the operational unit and the at least one spare unit comprise a combinational group of units, the at least one spare unit comprises a first spare unit in a cold standby state and a second spare unit in a hot standby state; express the one or more spare reliability metrics and the operational reliability metrics as one or more reliability metrics for the combinational group; calculate a plurality of transition values for the combinational group based on the one or more reliability metrics, each of the plurality of transition values indicating a corresponding probability of the combinational group transitioning from a first state of the operational state, the cold standby state, and the hot standby state to a second state of the operational state, the cold standby state, and the hot standby state, the calculating of the plurality of transition values comprising calculating a probability that the combinational group is to transition from the cold standby state to the hot standby state, or from the hot standby state to the cold standby state; calculate an availability value for the system based on the plurality of transition values, to calculate the availability value comprising adding one or more transition values of the plurality of transition values corresponding to probabilities of the combinational group being in the operational state, and refraining from adding other transition values corresponding to probabilities of the combination group being in the cold standby state or the hot standby state; and output the availability value to the user interface.