"Nines" is shorthand for availability levels. Three nines means 99.9%, four nines 99.99% and five nines 99.999%. Each additional nine cuts the allowed downtime by a factor of ten.
Why it matters
The shorthand makes targets easy to discuss, and the numbers show how fast difficulty and cost grow. Moving from 99.9% to 99.99% usually needs redundancy, automated failover and faster detection, because the margin for human response disappears.
Example
Allowed downtime per year (365 days) is roughly:
99% 3 days 15 hours 36 minutes
99.9% 8 hours 45 minutes 36 seconds
99.99% 52 minutes 34 seconds
99.999% 5 minutes 16 secondsAt five nines, one slow human response can use up the whole year.
Common mistakes
- Promising five nines for a single server in one data center.
- Forgetting that your availability is limited by dependencies. If you depend on three services at 99.9% each in series, the combined figure is about 99.7%.
- Ignoring the measurement window. A monthly target is much stricter than the same percentage per year.
- Not agreeing on what counts as downtime.
Related
Get notified when this term updates
Plus occasional practical tips on uptime and monitoring, nothing else. Unsubscribe anytime.
This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.