Uptime is the share of time a website or server is reachable. Hosts advertise it as a percentage, but the difference between "three nines" and "four nines" is larger than it looks, and an uptime figure only means something together with how it is measured.
Uptime percentages in real time
| Uptime | Downtime per month | Downtime per year |
|---|---|---|
| 99% | about 7 hours 18 minutes | about 3.65 days |
| 99.5% | about 3 hours 39 minutes | about 1.8 days |
| 99.9% | about 44 minutes | about 8 hours 46 minutes |
| 99.95% | about 22 minutes | about 4 hours 23 minutes |
| 99.99% | about 4 minutes | about 53 minutes |
What an SLA is
A service level agreement (SLA) is the host's formal promise, usually an uptime target with a compensation scheme such as service credits if it is missed. Read three things in any SLA:
- What is measured: the network, the server, or your actual website.
- Exclusions: scheduled maintenance, attacks, and problems caused by your own code are usually excluded.
- How to claim: credits are often only given on request within a set time.
Downtime that is not the host's fault
- An expired domain or wrong DNS records.
- A plugin update that breaks the site (a working server still serves an error page).
- Hitting your plan's resource limits; see resource limits explained.
- An expired SSL certificate or a redirect loop.
Monitor your own site
Use an external monitoring service that checks your site every minute or few minutes and alerts you by email or app. You can also run your own monitor on a VPS, see self-host Uptime Kuma. Monitoring tells you about problems before customers do, and gives you data if you ever need to claim under an SLA.