Technical Definition of Uptime
Uptime is a system reliability metric representing the percentage of time a service, application, or server remains fully operational and accessible to clients. It is the primary benchmark for measuring infrastructure health, calculated over monthly, quarterly, or yearly intervals.
Availability is calculated using the following formula:\[\text{Availability (\%)} = \left(\frac{\text{Total Time} - \text{Unplanned Downtime}}{\text{Total Time}}\right) \times 100\]For example, a service that experiences 43 minutes of unplanned downtime in a 30-day month achieves 99.9% availability.
The "Nines" of Uptime
- 99% (Two Nines): Up to 3.65 days of downtime per year.
- 99.9% (Three Nines): Up to 8.76 hours of downtime per year.
- 99.99% (Four Nines): Up to 52.56 minutes of downtime per year.
- 99.999% (Five Nines): Up to 5.26 minutes of downtime per year.