Server Uptime and SLAs: What 99.9% Really Means

What uptime percentage means

Uptime is the share of time a service is available. The gap between numbers looks small but is not: each extra nine cuts the allowed downtime by roughly ten times. Reading the percentage as "how much downtime is acceptable" makes the difference obvious.

Downtime per year, by the numbers

• 99% uptime: about 3 days, 15 hours of downtime per year.
• 99.5% uptime: about 1 day, 20 hours per year.
• 99.9% uptime: about 8 hours, 45 minutes per year.

So 99% and 99.9% are not almost the same, one allows more than three and a half days of yearly downtime, the other under nine hours. When a host quotes a number, check how many nines are actually after the decimal.

What an SLA actually promises

A Service Level Agreement is the contractual version of an uptime claim. It states the target, how it is measured, and what you get if the host misses it, usually a service credit on a future invoice. An SLA does not prevent downtime; it defines accountability when downtime happens.

Read the fine print

• How is uptime measured, at the network edge, or end to end?
• What is excluded, scheduled maintenance and events outside the host control usually are.
• How do you claim a credit, and how long do you have to do it?
• Is the status and incident history published openly, or hidden?

Transparency beats big numbers

A host that publishes its status page and real incident history is telling you more than one quoting an impressive number with no way to verify it. Corenzy targets 99.9% monthly uptime and publishes status incidents openly, so the SLA is something you can audit, not just a footnote.

An extra nine is not a rounding detail, it is the difference between hours and minutes of downtime a year. Always check how many nines, and whether the host proves them.