Term · Security & infrastructure
Uptime
Uptime is the proportion of time a website, application or service is available and working as expected, usually expressed as a percentage over a month or year. Its opposite is downtime.
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Explained
What the percentages mean
Availability targets sound similar but differ significantly in practice. Over a 30-day month:
| Availability | Maximum downtime per month |
|---|---|
| 99% | about 7 hours 12 minutes |
| 99.5% | about 3 hours 36 minutes |
| 99.9% | about 43 minutes |
| 99.99% | about 4 minutes |
These are arithmetic, not promises: they show what a given target allows.
How it is measured
External monitoring checks key pages or endpoints at regular intervals from several locations. "Available" should mean more than the home page responding: forms submit, payments work, the application logs in. See 24/7 monitoring.
What affects uptime
- Hosting reliability and redundancy.
- Software updates and deployments without testing.
- Traffic spikes and attacks. See CDN.
- Expired certificates and domains.
- Dependencies on third-party services.
Improving uptime
- Reliable hosting with redundancy for critical systems.
- Staging and tested deployments with rollback.
- CDN and DDoS protection.
- Monitoring with alerts and an on-call process.
- Tested backups for recovery.
Uptime targets are often written into a service level agreement.
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FAQ
Frequently asked questions
Is 100% uptime possible?
Not realistically. Even major platforms have incidents. Aim for a target that matches business impact and budget.
Does planned maintenance count as downtime?
It depends on the agreement. Many SLAs exclude announced maintenance windows.