Calculating Availability
Overview
Availability is measured in nines notation, which corresponds to the percentage of time that a service is available. A service that is available 99% of the time is said to have “two-nines availability.”
This notation is useful because it gives us a specific amount of downtime that a service is allowed to have. If your service is required to have four-nines availability, then it is allowed 52.56 minutes of downtime per year, which is 4.38 minutes of downtime per month, 1.01 minutes of downtime per week, and 8.66 - seconds of downtime per day
Here are the availability and downtime calculations for 99%- availability to 99.999% availability:
99% availability: (two-nines)
- 3.65 days/year (of allowed downtime)
- 7.20 hours/month
- 1.68 hours/week
- 14.4 minutes/day
99.9% availability (three-nines):
- 8.76 hours/year
- 43.8 minutes/month
- 10.1 minutes/week
- 1.44 minutes/day
99.99% availability (four-nines):
- 52.56 minutes/year
- 4.38 minutes/month
- 1.01 minutes/week
- 8.66 seconds/day
99.999% availability (five-nines):
- 5.26 minutes/year
- 25.9 seconds/month
- 6.05 seconds/week
- 864.3 milliseconds/day