Availability levels are expressed as "nines":

• 99% (Two 9s): 3.65 days downtime/year - Basic consumer services
• 99.9% (Three 9s): 8.77 hours downtime/year - Small business applications
• 99.99% (Four 9s): 52.6 minutes downtime/year - Enterprise applications
• 99.999% (Five 9s): 5.26 minutes downtime/year - Critical systems (telecom, financial)
• 99.9999% (Six 9s): 31.5 seconds downtime/year - Ultra-critical systems

Each additional "9" exponentially increases complexity and cost. Achieving five 9s requires redundant everything: power, network, hardware, software, and often geographically distributed infrastructure.