The availability of a website is measured by the percentage of a year in which the website is publicly accessible and reachable via the internet. This is different than measuring the uptime of a system. Uptime refers to the system itself being online, however it does not take into account being able to reach it as in the event of a network outage.
The formula to determine a system’s availability is relatively easy: Total time = 365 days per year * 24 hours per day * 60 minutes per hour = 525,600 minutes per year. To calculate how many minutes of downtime a system may experience per year, take the uptime guarantee and multiply it by total time in a year.
In the example of 99.99%: (1 - .9999) * 525,600 = 52.56 allowable minutes down per year.
The following table shows the translation from a given availability percentage to the corresponding amount of time a system would be unavailable per year, month, or week.
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