SLA Uptime Levels Explained

What's the difference between 99.9% and 99.99% uptime? More than you might think. Here's what each level means in real downtime.

The difference between 99.9% and 99.99% seems small—just 0.09%—but it's the difference between 8 hours of downtime per year and 52 minutes. Each additional "nine" represents a 10x improvement in reliability.

The "nines" explained

Level	Uptime %	Downtime/Year	Downtime/Month	Downtime/Week
Two nines	99%	3.65 days	7.3 hours	1.68 hours
Three nines	99.9%	8.76 hours	43.8 minutes	10.1 minutes
Three and a half nines	99.95%	4.38 hours	21.9 minutes	5.04 minutes
Four nines	99.99%	52.6 minutes	4.38 minutes	1.01 minutes
Five nines	99.999%	5.26 minutes	26.3 seconds	6.05 seconds

Which level do you need?

99.9% (Three nines)

The most common SLA for SaaS products and cloud services. Allows about 43 minutes of downtime per month—enough for one significant outage or several small hiccups.

Good for: Most web applications, APIs, internal tools, content sites

99.99% (Four nines)

Enterprise-grade reliability. Only 4.4 minutes of downtime per month. Requires significant investment in redundancy, failover, and incident response.

Good for: Payment systems, critical infrastructure, healthcare, financial services

99.999% (Five nines)

Near-continuous availability. Only 5 minutes of downtime per year. Extremely expensive to achieve—requires geographic redundancy, automatic failover, and 24/7 operations.

Good for: Emergency services, core banking, air traffic control

99% (Two nines)

Basic availability. Over 7 hours of downtime per month. Acceptable for non-critical systems, development environments, or services where occasional outages are tolerable.

Good for: Dev/staging, internal tools, non-critical batch jobs

The cost of each nine

Each additional nine typically requires 10x the investment in infrastructure, operations, and engineering:

99% → 99.9% – Basic redundancy, automated deployments, monitoring
99.9% → 99.99% – Multi-region deployment, automatic failover, 24/7 on-call
99.99% → 99.999% – Active-active multi-region, chaos engineering, dedicated SRE teams

Most companies find diminishing returns beyond 99.9%. The engineering cost to reach five nines often exceeds the business value—unless you're running truly critical infrastructure.

Real-world examples

Cloud providers

AWS EC2 – 99.99% monthly uptime SLA
Google Compute Engine – 99.99% monthly uptime SLA
Azure VMs – 99.95% to 99.99% depending on configuration

SaaS products

Stripe – 99.99% for core payment APIs
Twilio – 99.95% for voice and messaging
Slack – 99.99% for paid plans

Databases

Amazon RDS Multi-AZ – 99.95%
Google Cloud Spanner – 99.999% (regional) to 99.99999% (multi-regional)

Calculating your requirements

To determine what SLA level you need, consider:

Impact of downtime – How much does each minute of outage cost in revenue, reputation, or user trust?
Customer expectations – What do your customers expect and what are they paying for?
Contractual obligations – Do you have SLAs with your own customers?
Regulatory requirements – Some industries have mandated availability levels
Engineering investment – Can you afford to build and operate at that level?

Use our SLA calculator to see exactly what any uptime percentage means in downtime.