Availability is measured in so-called nines notation, that corresponds to the percentage of time that a service is available. For example, if a service is available 99.9% of the time, we can say that has “three-nines availability”. This notation is used to gives us the specific amount of downtime that a service is allowed to have.
Next are the availability and downtime calculations for 99% availability to 99.999% availability :
99% availability (two-nines):
* 3.65 days/year (of allowed downtime)
* 7.20 hours/month
* 1.68 hours/week
* 14.4 minutes/day
99.9% availability (three-nines):
* 8.76 hours/year
* 43.8 minutes/month
* 10.1 minutes/week
* 1.44 minutes/day
99.99% availability (four-nines):
* 52.56 minutes/year
* 4.38 minutes/month
* 1.01 minutes/week
* 8.66 seconds/day
99.999% availability (five-nines):
* 5.26 minutes/year
* 25.9 seconds/month
* 6.05 seconds/week
* 864.3 mimiseconds/day