What Is Uptime?
Defining Uptime and Downtime
What is uptime? It is the share of time a website or server is available and responding to requests, usually stated as a percentage over a month or a year. A site reachable for 719 hours of a 720-hour month has an uptime of about 99.86 percent for that month.
Downtime is the opposite: the time a site cannot be reached or does not work properly. A percentage hides how long each outage lasted, so it helps to convert it into minutes and hours.
What Each Percentage Allows
Small differences in the percentage matter a great deal over a year. A year has 525,600 minutes, so the downtime each level allows works out to roughly:
- 99%: about 3.65 days a year.
- 99.9%: about 8.8 hours a year, or about 43 minutes in a 30-day month.
- 99.99%: about 53 minutes a year, or about 4 minutes in a 30-day month.
Whether these differences matter depends on the site. A brochure site may tolerate a few hours of outage a year, while an online shop taking orders around the clock may not. Higher levels generally need more redundancy, which is covered in high availability hosting.
How Uptime Is Measured
Uptime is usually measured by external monitoring services. These request a page at regular intervals, often every minute, from several locations. A check counts as failed when the server does not answer, returns an error or responds too slowly. Uptime for a period is then the share of time without failed checks.
Results depend on method. A monitor in one region may see a problem that others miss. A check that loads only a small test page may not notice a broken checkout. A host's own figures may be calculated differently from a third-party measurement. Comparing uptime claims is most meaningful when the method and period are the same.
Frequently asked questions
What is a good uptime percentage for a website?
There is no single answer, but many hosting plans advertise 99.9% or higher. The right level depends on what an hour of outage costs the site and its owner. A personal blog can accept more downtime than an online store. Higher figures generally require more redundancy, which costs more to build. Contract terms are covered in hosting uptime guarantees.
Is 99% uptime good enough?
It depends on the site. At 99%, a site can be down for about 3.65 days a year, which is a lot for a business but may be acceptable for a hobby project. Where visitors or customers expect constant availability, a higher figure is usually needed. Check the measured history, not only the advertised number. Causes of outages are described in website downtime causes.
How can I check a website's uptime history?
Public status pages, third-party monitoring services and host-published reports can show past availability. Compare sources that use the same period and measurement method. Monthly summaries can hide short outages, so a detailed history gives a fuller picture.