What this calculator does
The tool calculates the probability that at least one path operates by subtracting the probability that every path fails. It is a reliability-block approximation for three redundant paths.
When to use it
Use it when a single operating path is sufficient and all paths are genuinely available on demand. Confirm switching logic, load-sharing behavior, and whether standby equipment can actually start and carry the duty.
Inputs and formula
Each input is reliability for the identical mission duration. Rsystem = 1 − (1−R1)(1−R2)(1−R3)
Worked example and interpretation
With reliabilities of 0.90, 0.80, and 0.70, the result is 99.4%. The high number reflects independent redundancy; it disappears quickly when a shared failure disables all paths.
Practical notes and limitations
Do not assume redundancy solves common utilities, common maintenance errors, common software, or demand failure. Dormant standby units may have a different failure model than continuously running units. Include proof-test and switchover performance where relevant.