The Uptime Institute’s Tier classification describes infrastructure redundancy (power, cooling):
- Tier I/II: single path, partial redundancy — outages during maintenance.
- Tier III: concurrently maintainable (N+1 components, dual paths) — the enterprise standard.
- Tier IV: fault-tolerant (2N, compartmentalization) — reserved for extreme needs.
You will find no per-Tier percentage here, and that is deliberate. “Tier III = 99.982%” is everywhere. Those figures come from a 2001 Uptime Institute document, where they described the best sites in a fleet that had actually been measured — not a promise attached to a class. The next edition filed them under a heading that says it all: attributes unrelated to Tier requirements. The institute later removed any reference to annual downtime. A Tier certifies a design, a capacity and an operation; it does not certify a rate. The only percentage you can hold a provider to is the one written in the contract, with its scope and its penalties.
Two buyer’s checks: many sites claim “Tier III equivalent” without certification — ask for the Uptime (or TIA-942) certificate and its expiry date; and remember that a Tier IV protects neither from human error nor from a regional disaster: real resilience comes from two distinct sites, not one perfect site.