What does 99.9% uptime mean (and what is a good SLA)?

99.9% uptime (three nines) allows about 8 hours 46 minutes of downtime per year, or roughly 43 minutes per month. Each extra nine cuts that by about ten times. What counts as good depends on what you are running: 99.9% is a common baseline for most SaaS, while infrastructure people depend on heavily often targets 99.95% or higher.

• 99% (two nines): about 3 days 15 hours of downtime per year.
• 99.9% (three nines): about 8 hours 46 minutes per year.
• 99.95%: about 4 hours 23 minutes per year.
• 99.99% (four nines): about 52 minutes per year.

Pick a target you can actually meet and measure, and be clear about what you are measuring (which components, and whether planned maintenance counts). An SLA you miss erodes more trust than a modest one you keep.