Understanding the “Nines of Availability”

If you’ve spent any amount of time in the tech field you’ve probably heard of the “Nines of Availability”. Availability is usually expressed as a percentage of uptime in a given year. The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously.

Availability %

Downtime per year

Downtime per month

Downtime per week

90% 36.5 days 72 hours 16.8 hours
99% 3.65 days 7.20 hours 1.68 hours
99.9% ("three nines") 8.76 hours 43.2 minutes 10.1 minutes
99.99% ("four nines") 52.6 minutes 4.32 minutes 1.01 minutes
99.999% ("five nines") 5.26 minutes 25.9 seconds 6.05 seconds
99.9999% ("six nines") 31.5 seconds 2.59 seconds 0.605 seconds
Posted by Brian 11/05/2009 Categories: High Availability