The Nines (Availability)
The nines is a shorthand used to express an availability target as a count of the 9 digits in its percentage: 99% is two nines, 99.9% is three nines, 99.99% is four nines, and 99.999% is five nines. Each additional nine represents a tenfold reduction in the amount of downtime allowed over a given period.
- Two nines, 99%: about 3.65 days of downtime per year
- Three nines, 99.9%: about 8.76 hours of downtime per year
- Four nines, 99.99%: about 52.6 minutes of downtime per year
- Five nines, 99.999%: about 5.26 minutes of downtime per year
This notation is a common shorthand inside Service Level Agreements, since 99.99% uptime is easier to state as a commitment than the exact number of minutes of allowed downtime. It is also used as a target for infrastructure and application design, driving decisions on redundancy, failover, and monitoring.
Reaching a higher number of nines becomes exponentially more expensive: each extra nine typically requires additional redundancy, more aggressive monitoring and alerting, and a matured incident-response process.