What Is Uptime?

Reviewed by Ionut Caval · Updated June 2026

Uptime is calculated as total time minus downtime, divided by total time, then multiplied by 100. If a service is unreachable for 30 minutes in a 30-day month (43,200 minutes), its uptime is (43,200 - 30) / 43,200 x 100, or about 99.93%. The figure is always tied to a window: 99.9% measured monthly is a stricter promise than 99.9% measured annually, because a single bad day weighs more heavily in a shorter period.

What the nines actually allow

Each extra nine cuts the allowed downtime by roughly a factor of ten, which is why the exact number matters far more than it looks. A figure like 99.9% sounds nearly perfect but still permits real outage time:

• 99% ("two nines") = about 7.2 hours of downtime per month, or 3.65 days per year.
• 99.9% ("three nines") = about 43 minutes per month, or 8.8 hours per year.
• 99.95% = about 22 minutes per month, or 4.4 hours per year.
• 99.99% ("four nines") = about 4.3 minutes per month, or 53 minutes per year.

For a fuller walkthrough of these thresholds, see what 99.9% uptime actually means, or convert any target into its allowed downtime with the uptime and SLA calculator.

How uptime is measured and used

Uptime is most meaningful when measured from outside your own infrastructure, the way a visitor connects, rather than from the server's own view of itself. A server can report itself as healthy while DNS, a CDN, an expired certificate, or a broken checkout makes the site unusable. This is why teams rely on synthetic monitoring, which runs scheduled external checks from multiple locations and counts a check as a failure when the response is missing, returns an error, or exceeds a set latency threshold.

Uptime is also the metric most often written into contracts. A Service Level Agreement (SLA) commits a provider to a specific uptime target over a stated window and defines remedies, such as service credits, if the target is missed. Because one extended outage can exhaust an entire monthly uptime budget, reliability depends not only on failing less often but on detecting and recovering fast when failures happen.

See also: Uptime & SLA calculator