Short version: an SLI is a measurement (e.g. the percentage of successful requests), an SLO is the internal target for that measurement (e.g. 99.9% over 30 days), and an SLA is the contractual promise you make to customers — usually a looser number than your SLO, with penalties if you miss it. You measure SLIs, you manage to SLOs, and you're held to SLAs.
SLI — Service Level Indicator
An SLI is a quantitative measure of some aspect of your service. Good SLIs reflect what users actually experience. Common ones:
- Availability: successful requests ÷ total requests.
- Latency: the share of requests faster than a threshold (e.g. 95% under 300 ms).
- Uptime: the share of time a monitored endpoint is reachable.
The key property of a good SLI is that it moves when users hurt. "CPU usage" is a metric, not a good SLI; "checkout success rate" is.
SLO — Service Level Objective
An SLO is the target you hold an SLI to, over a window. "99.9% availability over a rolling 30 days" is an SLO. SLOs are internal: they're how your team decides whether reliability is good enough to ship features or whether you should slow down and fix things.
The gap between your SLO and 100% is your error budget — the amount of unreliability you're allowed to spend. If your SLO is 99.9%, your error budget is 0.1%, which over a year is about 8 hours 45 minutes of downtime. Spend it on risky deploys; when it's gone, freeze and stabilize.
SLA — Service Level Agreement
An SLA is the promise you make to customers in a contract, with consequences (often service credits) when you break it. Crucially, your SLA should be looser than your SLO. If you operate to a 99.9% SLO internally, you might promise customers 99.5% — so you have headroom to miss your target without breaching a contract.
How they fit together
| Term | What it is | Audience | Example | | ---- | ------------------ | ----------------- | ------------------------ | | SLI | A measurement | Engineering | % of successful requests | | SLO | An internal target | Engineering | 99.9% over 30 days | | SLA | A customer promise | Legal / customers | 99.5%, or credits apply |
Why measurement granularity matters here
Every one of these depends on an SLI you can actually trust. If your monitor only checks every 30–60 seconds, a brief outage is either invisible or smeared across the polling window — which means your availability SLI is approximate, and any SLO or SLA built on it inherits that fuzziness. PingInsight's true 1-second checks record outages to the exact second, so the number you report is the number that happened.
Use the uptime calculator to see exactly how much downtime each SLO level allows, and read why second-by-second monitoring matters for the detection side of the story.