Achieving 99.999% ('Five Nines') availability allows for how much downtime per year?

99.999% availability (Five Nines) is an extremely high standard often required for critical healthcare, financial, or power grid systems. In a year of 31.5 million seconds, a 'Five Nines' target allows for only 315 seconds of downtime—just over 5 minutes *per year*. This leaves no room for human intervention during a failure; reaching this level requires full automation, redundant 'hot-standby' components across multiple physical locations, and rigorous automated testing. The cost of moving from 'Four Nines' (~52 minutes) to 'Five Nines' is often exponential, as it necessitates a level of engineering perfection that is very difficult to maintain.