What is availability?

Availability measures the percentage of time a system is operational and accessible to users. It is typically expressed as a percentage and is a core component of Service Level Objectives (SLOs) and Service Level Agreements (SLAs).

The formula is simple: Availability = (Uptime / Total Time) × 100

The allowed downtime is the inverse: Downtime = Total Time × (1 - Availability / 100)

The nine-nines

Availability targets are often described in terms of "nines" — the number of 9s in the uptime percentage. 99.9% is "three nines", 99.99% is "four nines", and so on. Each additional nine reduces allowed downtime by roughly 90%.

Note: Most consumer web services target 99.9% (three nines). Mission-critical infrastructure often targets 99.99% or 99.999%. True 100% availability is not achievable in practice — planned maintenance alone makes it mathematically impossible.

Availability vs reliability

Availability and reliability are related but distinct. Availability measures whether a system is up right now. Reliability measures how consistently it performs over time. A system can be highly available (rarely down) but unreliable (slow, error-prone when up).