The arithmetic

Over thirty days, 99.9% permits about 43 minutes of downtime, 99.99% about four minutes, and 99% nearly seven and a half hours. The gap between the first two is a different class of infrastructure.

What the guarantee usually excludes

Read the exclusions carefully:

  • Scheduled maintenance
  • Problems in your own code
  • Third-party network faults
  • DDoS mitigation periods

Measure it yourself

Independent monitoring from outside your host is the only figure you can act on. A provider measuring its own uptime has an obvious conflict of interest.