Uptime is a measure of the time a machine, system or application is operational and available to end users. This is usually expressed as a percentage (e.g. 99.99%). Uptime is a key metric used by many cloud hosting providers; the distributed spread of servers used in cloud computing means that downtime (the time in which a resource is inoperative) is extremely rare.

Many well-known websites have experienced downtime, which can often be caused by sudden surges in traffic. Even Twitter suffered a famous crash in 2014 as a result of the iconic “Oscars selfie”. Cloud hosting is one way of ensuring near-100% uptime for websites and applications: multiple servers means no single point of failure, while you can quickly scale resources as traffic increases.
Downtime is almost never a good thing (even when it’s planned for), so ensuring uptime remains as close as possible to 100% is vitally important for any business. If an ecommerce website was to experience significant downtime, for example, it would likely lead to considerable loss of revenue for the organization.
Uptime is calculated based on the number of hours a server is available during a specified period of time, and is normally expressed as a percentage. For example, annual uptime is calculated by dividing the number of hours of uptime by the total number of hours in a year, and multiplying by 100. There are 8,760 hours in a year (365 x 24 = 8760). If there were 4 hours of downtime during the year, that means a total of 8,756 hours of uptime. To get the annual uptime percentage, you’d do the following calculation: (8756 / 8760) x 100 The answer to above equation is 99.9543378995, which means the annual uptime percentage would equal 99.95%.