What is uptime?
Uptime is the share of time a website or service is reachable and working, usually shown as a percentage of a period such as a month.
In more detail
Uptime is measured by checking the site from outside at regular intervals and counting the time it was down. Two choices change the number a lot: which failures count (a single failed check, or only failures confirmed by a second check) and what is excluded (planned maintenance, for example).
Uptime is often quoted in "nines". Each extra nine cuts the allowed downtime by ten: 99% allows about 7 hours 12 minutes in a 30-day month, 99.9% about 43 minutes, 99.99% about 4 minutes.
Worked example
A 30-day month has 43,200 minutes. A site that was down for one outage of 70 minutes had 43,130 minutes up, so its uptime was 43,130 ÷ 43,200 = 99.838%. That misses a 99.9% target, which allows 43.2 minutes.
OwlHound counts only confirmed outages, those where the next check round failed too, and leaves out time inside a maintenance window you set.
See also
Related terms: Downtime and SLA (service level agreement). Free tool: Uptime calculator. How OwlHound helps: Website uptime monitoring. All terms: monitoring glossary.
Frequently asked questions
Is 99.9% uptime good?
It is a common target. It allows about 43 minutes of downtime in a 30-day month, or about 8 hours 46 minutes in a year.
How is uptime calculated?
Minutes the site was up, divided by the minutes in the period, times 100. The minutes it was down come from monitoring checks, so the check interval and the rule for what counts as down both matter.
Does planned maintenance count as downtime?
It depends on who is counting. Many SLAs exclude announced maintenance. OwlHound does not count time inside a maintenance window you set.
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