Short answer: Uptime is the share of time a server or service is reachable, usually quoted monthly as a percentage: 99.9% allows about 43 minutes of downtime a month, 99.95% about 22 minutes and 99.99% about 4 minutes.
Uptime explained
Providers measure uptime at the network or hypervisor level, so your application can be down while the host still counts the server as up. Independent monitoring of your own service is the only way to know.
Redundant power and network, replicated storage and live migration are what let providers promise higher figures.
At a glance
| Uptime | Downtime per month | Downtime per year |
|---|---|---|
| 99% | ≈7.3 hours | ≈3.65 days |
| 99.9% | ≈43.8 minutes | ≈8.8 hours |
| 99.95% | ≈21.9 minutes | ≈4.4 hours |
| 99.99% | ≈4.4 minutes | ≈53 minutes |
See it in the data
Frequently asked questions
What uptime should a VPS have?
Good providers deliver well above 99.9% in practice; the SLA shows what they are willing to guarantee in writing.