What is uptime?
Uptime is the time a service is available and working, usually reported as a percentage of a period: availability = uptime ÷ (uptime + downtime). A site that was down for 26 minutes in a 30-day month had 99.94% uptime. It is measured by external checks at a regular interval, so the interval limits how precisely downtime is counted.
Why uptime matters
Uptime is the number customers understand and the one SLAs are written in. Tracking it per service shows whether reliability is improving and whether you can keep the promises in your contracts.
The nines hide large differences: 99% allows 7.2 hours of downtime in a 30-day month, 99.9% allows 43.2 minutes and 99.99% only 4.32 minutes.
How to measure uptime
availability = uptime ÷ (uptime + downtime) × 100% 30-day month = 43,200 minutes
Worked example
A site was down twice in a 30-day month, for 19 and 7 minutes.
- Downtime = 19 + 7 = 26 minutes; uptime = 43,200 − 26 = 43,174 minutes.
- Availability = 43,174 ÷ (43,174 + 26) = 99.94%.
- That meets a 99.9% target (43.2 minutes allowed) but not 99.95% (21.6 minutes allowed).
Result: The same month reads as a pass or a fail depending on the target, so publish the minutes as well as the percentage.
Common mistakes
- Checking only the homepage. The homepage can be up while login, the API or checkout are down.
- Long intervals. With 15-minute checks a 5-minute outage may never be seen, and one failed check counts as 15 minutes.
- Counting unconfirmed failures. A single network blip is not downtime. Confirm with a retry.
- Calling degraded "up". A page that takes 30 seconds is down for users. Add response-time limits.
How Vigil handles uptime
- Vigil's Uptime monitor checks your URL at the interval you choose, confirms failures with a second check about 12 seconds later, keeps evidence of each failure, and reads the TLS certificate on every check.
- Uptime per monitor shows on dashboards, in the weekly report and on status pages as 90-day bars; the SLA report prints it against a target.
- Try the free uptime checker, or convert a percentage to minutes with the SLA calculator.
Uptime: frequently asked questions
What is the difference between uptime and availability?
They are often used as synonyms. Strictly, uptime is the time a system is running, while availability is whether users can use it; a server can be up while the application on it is unavailable.
How much downtime is 99.9% uptime?
43.2 minutes in a 30-day month and about 8.76 hours in a 365-day year.
How often should uptime be checked?
Every 1 to 5 minutes for important services. The interval is both your worst-case detection time and the resolution of your downtime numbers.