This is very important to understand what is high availability 9′s, This is a measurement of high availability  where you can tell how much downtime is feasible for your organization for a Year(annually).

the calculation is as follows

=Total Hours in a Year-(Total Hours in a Year* # of 9′s)/100

where Total Hours in a Year =365(days)*24 Hours =8760 Hours

so in this way if you have 2 -9′s ie. 99% the down time duration could be.

=8760-(8760*99)/100

=87.6 hours

=3 Days and 15 Hours

simillarly if you have 3 – 9′s means 99.9%

=8760-(8760*99.9)/100

=8.76 hours (8 hours and 45minutes)

and so on.