Uptime guide & SLA calculator

What do “three nines” actually mean?

Availability is usually stated as a percentage of uptime over a period — most often a year. The phrase “number of nines” is shorthand for how many 9s appear after the decimal point: 99.9% is “three nines”, 99.99% is “four nines”, and so on.

The gap between these numbers sounds tiny but is enormous in practice. A year has 525,600 minutes, so each extra nine removes 90% of the remaining allowed downtime. At 99.9% a service may be down for about 8.8 hours a year; at 99.99% that drops to roughly 52.6 minutes; at 99.999% it is only about 5.3 minutes. That is the difference between a service that is merely “very reliable” and one engineered for near-continuous operation.

An SLA (service-level agreement) is a contractual promise that formalizes this: it states the availability target, how it is measured, and what happens — usually service credits — if the provider misses it. SLAs often also define exclusions such as planned maintenance, and they typically measure availability over a month rather than a year. Always read the fine print of any SLA before relying on it.

To go deeper, open the status board or the incident list.