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

Last updated 23 June 2026

99.9% uptime (three nines) allows about 8 hours 46 minutes of downtime a year, or about 43 minutes a month. Each extra nine divides that by ten. Many SaaS products commit to 99.9%. Infrastructure that other services build on 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 meet and measure, and write down what it covers: which components, and whether planned maintenance counts. Customers remember a missed SLA longer than a modest one that was kept.

Monitor your services and tell customers when something breaks.

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