Understanding Percentage Availability
Percentage availability is a key metric used to measure the reliability of systems, services, and infrastructure. It represents the proportion of time a system is operational and accessible.
Formula
The availability percentage is calculated using the following formula:
[
\text{Availability} = \frac{\text{Total Uptime}}{\text{Total Uptime} + \text{Total Downtime}} \times 100
]
The result is expressed as a percentage (%).
Where:
- Total Uptime is the time the system was operational (in hours)
- Total Downtime is the time the system was not operational (in hours)
Example Calculation
If a server has been running for 720 hours with 25 hours of downtime:
[
\text{Availability} = \frac{720}{720 + 25} \times 100 = \frac{720}{745} \times 100 = 96.65
]
So the availability is 96.65%.
Common Availability Targets
- 99% - Allows approximately 3.65 days of downtime per year
- 99.9% (Three Nines) - Allows approximately 8.76 hours of downtime per year
- 99.99% (Four Nines) - Allows approximately 52.6 minutes of downtime per year
- 99.999% (Five Nines) - Allows approximately 5.26 minutes of downtime per year
Applications
Availability calculations are commonly used in:
- Service Level Agreements (SLAs) - Defining uptime commitments
- IT Infrastructure - Measuring server and network reliability
- Manufacturing - Tracking equipment operational time
- Cloud Services - Monitoring service uptime guarantees