What is SLA vs SLO vs SLI?

April 21, 2026 • 3 min read

SLA, SLO, and SLI are often confused.

But they are not the same thing.

Understanding the difference is essential if you want to build reliable systems.

TL;DR

• SLI = what you measure
• SLO = the target you aim for
• SLA = the promise you make

What is an SLI?

SLI stands for Service Level Indicator.

It is a metric that measures the performance of your system.

Examples:

• uptime percentage
• response time
• error rate

An SLI answers the question:

👉 What are we measuring?

What is an SLO?

SLO stands for Service Level Objective.

It is the target value for your SLI.

Examples:

• 99.9% uptime
• response time under 300ms
• error rate below 1%

An SLO answers:

👉 What is acceptable performance?

What is an SLA?

SLA stands for Service Level Agreement.

It is a formal commitment made to users or customers.

It often includes:

• guaranteed targets (based on SLOs)
• consequences if not met (credits, refunds)

An SLA answers:

👉 What are we promising to users?

How they work together

These three concepts are connected:

• SLI → what you measure
• SLO → what you aim for
• SLA → what you promise

Example:

• SLI: uptime
• SLO: 99.9% uptime
• SLA: refund if uptime drops below 99.9%

Why this matters

Without clear definitions:

• you don’t know what to measure
• you don’t know what “good” means
• you can’t communicate reliability

These concepts turn vague ideas into measurable goals.

The common mistake

Many systems focus only on uptime.

But reliability is more than a single metric.

It also depends on:

• consistency
• accurate monitoring
• avoiding false alerts

👉 To understand how uptime is measured, read what is uptime

From metrics to reality

Having SLOs is not enough.

You need reliable monitoring to measure them correctly.

If your alerts are noisy:

• your data becomes unreliable
• your SLOs lose meaning

👉 Learn how monitoring works in what is uptime monitoring

Next step

Now that you understand reliability metrics, the next step is seeing how users experience them.

👉 Read what is downtime

Try it in practice

PulsorUp helps you track real reliability signals.

Instead of reacting to every failure, it confirms issues before alerting.

Metrics define reliability.

Accurate signals make them useful.