Three Nines (99.9%) SLA Explained
Understand what a 99.9% SLA (three nines) means in terms of allowed downtime per year, month, and day. Common for standard cloud services and web applications.
Detailed Explanation
What Does 99.9% Uptime Mean?
A 99.9% SLA, commonly called three nines, is one of the most widely used availability targets in the cloud computing industry. It allows a total of approximately 8 hours and 46 minutes of downtime per year.
Downtime Breakdown
| Period | Allowed Downtime |
|---|---|
| Per year | 8 hours, 45 minutes, 57 seconds |
| Per month | 43 minutes, 50 seconds |
| Per week | 10 minutes, 5 seconds |
| Per day | 1 minute, 26 seconds |
Why Three Nines?
Three nines strikes a balance between cost and reliability. Achieving higher availability (four or five nines) requires significantly more investment in redundancy, monitoring, and operational processes. For many web applications, 43 minutes of monthly downtime is acceptable — especially when some of that can be scheduled during low-traffic hours.
Who Offers Three Nines?
Most single-instance cloud deployments (a single VM, a single-AZ database) come with a 99.9% SLA:
- AWS: Single-instance EC2, single-AZ RDS
- Azure: Single-instance VMs with Premium SSD
- GCP: Single-instance Compute Engine
Error Budget at Three Nines
With a 0.1% error budget per month (~43.8 minutes), an SRE team can afford roughly one moderate incident per month. If a deployment causes 10 minutes of errors, that consumes about 23% of the monthly error budget, leaving room for unexpected issues.
Use Case
Use three nines as a baseline SLA for standard web applications, internal tools, and services where brief downtime during off-peak hours is acceptable. Most SaaS products start with a 99.9% SLA in their terms of service.