Percentage calculation Availability %Downtime per yearDowntime per month 99.9% ("three nines")8.77 hours43.83 minutes 99.95% ("three and a half nines")4.38 hours21.92 minutes 99.99% ("four nines")52.60 minutes4.38 minutes 99.995% ("four and a half nines")26.30 minutes2.19 minutes 12 more rows