System uptime is a measure of how much time any given system is up, running, accessible and doing what it is supposed to do. In our case, we are concerned with the uptime of our web server and the network services which connects it to the Internet. Uptime is expressed as a percentage. The amount of uptime that is considered acceptable will vary from one situation to another. For example, if your website is mainly informational, you would probably be more tolerant of downtime than someone who has an eCommerce site that sells products and thus generates income directly from their website.

Is 99% uptime good? It sounds like a good percentage, but consider that if our website is up 99% of the year, our web pages were not available for 3.7 days over the course of the year! That's not so good. If our uptime increases to 99.9% our downtime is only 8.8 hours per year. At 99.99%, downtime is only 52.6 minutes out of a year.

When we get to 99.999% (the five 9s) downtime is only 5.3 minutes per year! This is the often sought after, seldom achieved uptime goal. Beware of any hosting company that claims they have 100% uptime over the course of a year; this goal is almost never achieved!