Glossary
five nines
99.999% availability — about five minutes of downtime per year. The aspirational target for critical infrastructure.
“Five nines” is shorthand for 99.999% availability — at most 5 minutes and 15 seconds of downtime in a calendar year, or about 26 seconds in a month. It’s the aspirational availability target for genuinely critical infrastructure: telephone networks, payment switches, control systems.
Most internet services do not achieve five nines. Even sophisticated cloud providers struggle to commit to four nines (99.99% — about 52 minutes a year) in their public SLAs. The number is included in the glossary because customers frequently ask for it and it’s worth being honest about what it costs and what it actually buys.