How many tiers are there of data centers?

The Uptime Institute has defined four different tiers of data centers (this definition is most commonly used):

• Tier I (99.67% availability) ~29 hours downtime in a year
• Tier II (99.74% availability) ~23 hours downtime in a year
• Tier III (99.98% availability) ~1.58 hours downtime in a year
• Tier IV (99.995% availability) ~ 0.438 hours downtime in a year

In practice, Tier II and Tier III data centers are the most common.