Glossary · Reliability metrics
Service level indicator
A service level indicator (SLI) is a measurement of one aspect of a service’s reliability, usually the share of good events: successful requests, checks that passed, or responses faster than a threshold.
Common SLIs are availability (successful requests ÷ all valid requests), latency (requests served faster than, say, 500 ms ÷ all requests), and freshness or correctness for data pipelines. As a ratio, every SLI runs from 0 to 100% and can be compared with an SLO.
SLIs come from server logs, load balancers or synthetic checks outside your network. Logs see every real request but miss users who never reached you, for example during a DNS or CDN failure; external checks see those failures but sample at an interval. Many teams use both.
Use percentiles, not averages, for latency SLIs: an average hides the slow tail that a few percent of users get.
SLI = good events ÷ valid events × 100
Example: 43,190 of the 43,200 one-minute checks in 30 days passed: 43,190 ÷ 43,200 × 100 = 99.976%.
Uptimecheck records every check with its result and timings, so availability (the share of monitored time without a confirmed outage) and response time percentiles are ready to use. Response times are split into DNS, connect, TLS, server and download, and charts show the median and the 95th percentile.
14-day free trial · No credit card required
Guides 7 min read
An SLI measures how a service behaves, an SLO is the target for that measurement and an SLA is the promise to customers. Definitions, downtime tables and how to set each one.
ReadMonitoring 7 min read
Response time is how long your server takes to answer a request, not how long the page takes to load. What counts as good, why monitors and browsers measure differently, and what slows sites down.
ReadYes, the most common one: the share of time, or of checks, in which the service worked.
An average of 200 ms can hide that 5% of users wait 3 seconds. The 95th percentile is the value 95% of requests were faster than, so it shows the slow tail.
As close to the user as practical. Server-side metrics miss failures in DNS, CDNs and networks before a request reaches you; external checks catch those.
Set up your first monitor in under a minute, or look around the live demo first.
14-day free trial · No credit card required
With your OK, Google Analytics and Microsoft Clarity show us which pages help and where visitors get stuck. They set cookies for up to 13 months. Without your OK, neither of them runs. You can change your mind at any time under Cookie settings at the bottom of the page. Privacy policy