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Reliability and uptime on web hosting

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. Thus we can get an idea of the time utilized.

Availability % Downtime per year Downtime per month* Downtime per week
90% (“one nine”) 36.5 days 72 hours 16.8 hours
95% 18.25 days 36 hours 8.4 hours
97% 10.96 days 21.6 hours 5.04 hours
98% 7.30 days 14.4 hours 3.36 hours
99% (“two nines”) 3.65 days 7.20 hours 1.68 hours
99.5% 1.83 days 3.60 hours 50.4 minutes
99.8% 17.52 hours 86.23 minutes 20.16 minutes
99.9% (“three nines”) 8.76 hours 43.2 minutes 10.1 minutes
99.95% 4.38 hours 21.56 minutes 5.04 minutes
99.99% (“four nines”) 52.56 minutes 4.32 minutes 1.01 minutes

*For monthly calculations, a 30-day month is used.

A hosting provider’s SLAs may include a certain amount of scheduled downtime per year in order to perform maintenance on the systems. This scheduled downtime is often excluded from the SLA timeframe, and needs to be subtracted from the Total Time when availability is calculated. Depending on the verbiage of an SLA, if the availability of a system drops below that in the signed SLA, a hosting provider often will provide a partial refund for time lost.