Five Nines

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Five Nines is the term used for describing the availability of a computer or a service at 99.999 percent of the time it is required. In other words, the system or service is only unavailable for 5.39 minutes throughout the year for planned or unplanned downtime. Five nines is recommended and required for mission-critical requirements and for certain areas such as e-commerce. However, five nines availability has always been a challenge for a service or network and is often impossible to guarantee.

Five nines in terms of availability

Number of Nines	percent	Availability Per Year
1	9%	332+ days of downtime per year
2	99%	3 days, 15 hours and 40 minutes downtime per year
3	99.9%	8 hours, 46 minutes downtime per year
4	99.99%	52 minutes, 36 seconds downtime per year
5	99.999%	5 minutes, 15 seconds or less of downtime in a year

Source: Five Nines