System Availability Calculator

For components in series (all must work for the system to work), system availability = product of all individual availabilities. If a web server has 99.9% availability and a database has 99.9% availability, the system availability = 0.999 * 0.999 = 99.80%. Adding more serial components always reduces overall system availability.

For parallel redundant components (system works if any one works), the probability of the subsystem being down = product of all component failure probabilities. If one component has 99.9% availability (0.1% failure probability) and we add an identical redundant component, subsystem failure probability = 0.001 * 0.001 = 0.000001, giving 99.9999% availability for that subsystem.

Availability measures the fraction of time a system is operational (uptime / total time). Reliability measures the probability that the system operates without failure for a specific duration. A highly reliable system (long time between failures) can still have low availability if it takes a long time to repair after each failure. Availability = MTBF / (MTBF + MTTR).

Inherent availability considers only corrective maintenance downtime (MTBF and MTTR), assuming no delays for parts or logistics. Operational availability includes all downtime: corrective maintenance, preventive maintenance, supply delays, and administrative delays. Operational availability is always lower than inherent availability and better represents real-world service delivery.

Model your system as a series-parallel network where each subsystem is a node. Calculate the availability of each subsystem considering its internal redundancy. Then multiply serial subsystem availabilities. Identify which subsystem is the bottleneck (lowest availability) - this is where additional redundancy will have the greatest impact on overall system availability.