Uptime Institute Tier levels (I-IV)

Uptime Institute Tier levels are a four-level classification of data center infrastructure resilience, from Tier I (basic capacity) to Tier IV (fault tolerant). Tier I has no redundancy and implies about 99.671% availability (up to 28.8 hours of downtime per year); Tier II adds redundant components (99.741%); Tier III is concurrently maintainable, meaning any component can be serviced without shutting down IT load (99.982%, about 1.6 hours/year); Tier IV tolerates any single equipment failure or distribution path outage (99.995%, about 26 minutes/year). Only Uptime Institute issues official Tier certifications, awarded separately for design documents, constructed facility, and operations. Most commercial colocation is built to Tier III or 'Tier III-equivalent' standards; certified Tier IV facilities are rare because 2N infrastructure roughly doubles capital cost.