The level of availability can be expressed by the Nine Rule :

Percentage of availability	Percentage of unavailability	Annual unavailability	Weekly unavailability
98%	2%	7.3 days	3 hours, 22 minutes
99%	1%	3.65 days	1 hour, 41 minutes
99.8%	0.2%	7 hours, 13 minutes	20 minutes, 10 secondes
99.9%	0.1%	8 hours, 45 minutes	10 minutes, 5 secondes
99.99%	0.01%	52.5 minutes	1 minute
99.999%	0.001%	5.25 minutes	6 secondes
99,9999%	0,0001%	31,5 seconds	0,6 secondes

The effect of service unavailability time can be more or less important according to the business and the day this happens. In most environments, the transition from a system availability of 99% to 99.9% increases the costs by 5 to 10 times.

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