Six Nines SLA
99.9999% Uptime Calculator

"Six nines" refers to 99.9999% availability - the percentage contains six 9 digits. It is well beyond what almost any commercial SLA offers and is mostly a theoretical or specialized engineering target.

99.9999% SLA allows about 31.5 seconds of downtime per year (using a 365-day year), or roughly 86.4 milliseconds per day - shorter than a single network timeout.

Telecom carrier-grade switching hardware, air traffic control systems, and certain safety-critical industrial controllers are among the rare systems engineered toward six nines. It is essentially never offered as a standard commercial cloud or SaaS SLA.

For most software systems, it is effectively theoretical. Achieving 31.5 seconds of annual downtime requires eliminating nearly every category of failure - hardware, network, human error, and software bugs - simultaneously, which is only cost-justified in a small number of specialized, purpose-built systems.

Six nines allows roughly 10 times less downtime than five nines: about 31.5 seconds per year versus about 5 minutes 15 seconds. That final nine is disproportionately expensive relative to the marginal reliability gained.