How is cloud service availability commonly quantified?

Cloud service availability is commonly quantified using "nines," which represent the percentage of uptime a service is expected to provide. This terminology comes from the notion of measuring the reliability of services in decimal terms. For instance, "three nines" (99.9% availability) means that a service could be down for approximately 8.76 hours in a year. "Five nines" (99.999%) is often considered the gold standard for critical services, indicating less than 5.26 minutes of downtime in a year. This metric allows organizations to assess the reliability of cloud services and helps in capacity planning and setting service level agreements (SLAs).

Other methods of quantifying availability, such as considering hours of uptime, do not provide the same standardized metric that can be easily compared across different services or providers. Similarly, user accounts and geographical locations do not reflect service continuity or uptime effectively. The use of "nines" allows for a clear and concise communication of reliability expectations among stakeholders.