Uptime is the measure of how reliably a server, service or website remains operational and accessible. It is expressed as a percentage and is a critical metric in any web hosting or cloud service agreement. The complement — downtime — is the time the system is unavailable.
Uptime SLA Reference
- 99% — Up to 87.6 hours downtime per year.
- 99.9% — Up to 8.76 hours downtime per year. Standard hosting SLA.
- 99.99% — Up to 52.56 minutes downtime per year. "Four nines" — enterprise level.
- 99.999% — Up to 5.26 minutes downtime per year. "Five nines" — mission-critical systems.
Causes of Downtime
- Hardware failure on the host server.
- Network outages or DDoS attacks.
- Software bugs or failed deployments.
- Scheduled maintenance windows.
- Resource exhaustion (out of memory, disk full).
Downtime and SEO
Brief downtime (minutes) rarely affects rankings. Extended downtime (hours) can cause Googlebot to return 503 errors and temporarily de-index your pages. If Googlebot sees persistent unavailability, it may reduce crawl frequency and eventually drop pages from the index.