What is downtime (and how to measure it)?

April 21, 2026 • 2 min read

Downtime is inevitable.

No system stays online forever.

But how you detect and handle downtime makes all the difference.

TL;DR

• Downtime is when your service is unavailable
• It can be planned or unplanned
• Even short outages can impact users and revenue
• Measuring downtime helps you understand reliability

What is downtime?

Downtime is any period when your service is not accessible to users.

If your website or API fails to respond correctly, it is considered down.

Even brief interruptions count as downtime.

Planned vs unplanned downtime

Not all downtime is the same.

Planned downtime

This happens when you intentionally take your system offline:

• deployments
• maintenance
• infrastructure changes

Unplanned downtime

This is the real problem.

It happens unexpectedly:

• server crashes
• network failures
• bugs in production

Unplanned downtime is what impacts users the most.

How to measure downtime

Downtime is measured as the total time your service is unavailable within a given period.

For example:

• Total time: 30 days
• Downtime: 1 hour

This means your service was unavailable for 1 hour during that period.

👉 To understand how this affects uptime, see what is uptime.

Why downtime matters

Downtime directly affects:

• user experience
• trust
• revenue

Even a few minutes of downtime can lead to lost users.

The problem with detecting downtime

Detecting downtime is not as simple as it sounds.

A single failed request does not always mean your system is down.

Temporary issues can happen:

• network instability
• slow responses
• brief outages

If you alert on every failure, you create noise.

Reliable downtime detection

Good monitoring systems confirm downtime before alerting.

Instead of reacting to a single failure, they:

1. detect an issue
2. retry the request
3. confirm the failure
4. then alert

This reduces false alarms and improves reliability.

👉 Learn how this works in what is uptime monitoring.

Next step

Now that you understand downtime, the next step is defining reliability goals.

👉 Read what is SLA vs SLO vs SLI

Try it in practice

PulsorUp helps you detect real downtime without false alerts.

Instead of notifying on the first failure, it confirms issues before sending alerts.

Downtime will always happen.

Reliable systems are the ones that detect it correctly.