Uptime describes how long or how reliably a system has been running. It may be defined as an absolute value (e.g., 64 days) or a percentage (e.g., 99.5%). Uptime percentage is a common metric used to determine the reliability of a web server.

Absolute uptime measures the length of time a system has been running since it was booted. For example, if a system was rebooted at 1:00 PM and is still running at 3:30 PM the next day, its uptime will be 26.5 hours. When a system is restarted, the uptime number resets.

On a Unix-based system (including macOS), you can view the system uptime by typing the uptime command in the terminal.

Uptime percentage is determined by dividing the total time has been available (or online) by the time it has been active. For example, if a web server has been active for one year, but was not available for a total of 32 hours during the year, its uptime percentage would be calculated as follows:

365 days x 24 hours = 8,760 active hours
8,760 active hours - 32 hours unavailable = 8,728 available hours
8,728 available hours ÷ 8,760 active hours = 99.6347% uptime

An uptime of 99.9% is a reasonable goal for a web server. That percentage may sound high, but even a few hours of downtime can have a significant impact on a business. For this reason, a high-traffic enterprise server may have an uptime goal of 99.999%. This level of uptime can be achieved by using multiple servers for redundancy and load balancing.

Updated December 31, 2018 by Per C.

Definitions by TechTerms.com

The definition of Uptime on this page is an original TechTerms.com definition. If you would like to reference this page or cite this definition, you can use the green citation links above.

The goal of TechTerms.com is to explain computer terminology in a way that is easy to understand. We strive for simplicity and accuracy with every definition we publish. If you have feedback about the Uptime definition or would like to suggest a new technical term, please contact us.

Want to learn more tech terms? Subscribe to the daily or weekly newsletter and get featured terms and quizzes delivered to your inbox.

Sign up for the free TechTerms Newsletter

How often would you like to receive an email?

You can unsubscribe or change your frequency setting at any time using the links available in each email.

Questions? Please contact us.