What does [xx]% uptime mean in reality?


SLA’s (Service-level agreements) quite often contain promises like ‘95% uptime’, ‘99% uptime’ or ‘99.99% Uptime SLA’. But what does this mean in reality?
Here’s a table with the most common values of high availability:

Availability %Downtime per yearDowntime per monthDowntime per week
90%36.5 days72 hours16.8 hours
95%18.25 days36 hours8.4 hours
99%3.65 days7.20 hours1.68 hours
99.5%1.83 days3.60 hours50.4 minutes
99.9%8.76 hours43.8 minutes10.1 minutes
99.95%4.38 hours21.56 minutes5.04 minutes
99.99%52.56 minutes4.32 minutes1.01 minutes
99.999%5.26 minutes25.9 seconds6.05 seconds
99.9999%31.5 seconds2.59 seconds0.605 seconds

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