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Tip: Calculating Availability

View the calculations for service availability for 99%, 99.9%, 99.99% and 99.999%.


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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

 

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Octavia Andreea Anghel is a senior PHP developer currently working as a primary trainer for programming teams that participate at national and international software-development contests. She consults on developing educational projects at a national level. She is a coauthor of the book "XML Technologies--XML in Java" (Albastra, ISBN 978-973-650-210-1), for which she wrote the XML portions. In addition to PHP and XML, she's interested in software architecture, web services, UML, and high-performance unit tests.
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