- Availability is usually measured as a percentage of uptime over total time.
- Common availability levels:
- 99% → ~3.65 days downtime/year
- 99.9% (three nines) → ~8.76 hours/year
- 99.99% (four nines) → ~52.6 minutes/year
- 99.999% (five nines) → ~5.26 minutes/year
- Higher availability requires more redundancy and failover systems.
- Formula: Availability = (Uptime / Total Time) × 100
- Even small improvements in availability need significant engineering effort.