Availability Table

Availability is generally calculated based on how long a service was unavailable over some period. Assuming no planned downtime, Table 1-1 indicates how much downtime is permitted to reach a given availability level.

Table 1-1. Availability table
Availability level Allowed unavailability window

 

per year

per quarter

per month

per week

per day

per hour

90%

36.5 days

9 days

3 days

16.8 hours

2.4 hours

6 minutes

95%

18.25 days

4.5 days

1.5 days

8.4 hours

1.2 hours

3 minutes

99%

3.65 days

21.6 hours

7.2 hours

1.68 hours

14.4 minutes

36 seconds

99.5%

1.83 days

10.8 hours

3.6 hours

50.4 minutes

7.20 minutes

18 seconds

99.9%

8.76 hours

2.16 hours

43.2 minutes

10.1 minutes

1.44 minutes

3.6 seconds

99.95%

4.38 hours

1.08 hours

21.6 minutes

5.04 minutes

43.2 seconds

1.8 seconds

99.99%

52.6 minutes

12.96 minutes

4.32 minutes

60.5 seconds

8.64 seconds

0.36 seconds

99.999%

5.26 minutes

1.30 minutes

25.9 seconds

6.05 seconds

0.87 seconds

0.04 seconds

Using an aggregate unavailability metric (i.e., "X% of all operations failed") is more useful than focusing on outage lengths for services that may be partially available—for instance, due to having multiple replicas, only some of which are unavailable—and for services whose load varies over the course of a day or week rather than remaining constant.

See Equations Time-based availability and Aggregate availability in Embracing Risk for calculations.