SLA Downtime Calculator

Calculate allowable downtime from uptime targets across day, week, month, quarter, and year windows.

Understanding Uptime & SLA

Uptime is a measure of system reliability, expressed as the percentage of time that a system is operational and available. Service Level Agreements (SLAs) use uptime targets to define contractual commitments between service providers and customers.

Common Uptime Targets

99% Uptime ("Two Nines")

Allows approximately 7.2 hours of downtime per month. Suitable for:

Internal tools and applications
Non-critical business systems
Development and testing environments

99.9% Uptime ("Three Nines")

Allows approximately 43.8 minutes of downtime per month. Common for:

Standard production systems
Business applications
Most web services and APIs
E-commerce platforms

99.95% Uptime

Allows approximately 21.9 minutes of downtime per month. Used for:

High-availability production systems
Customer-facing applications
Premium service tiers

99.99% Uptime ("Four Nines")

Allows approximately 4.38 minutes of downtime per month. Required for:

Mission-critical systems
Financial services platforms
Healthcare applications
Telecommunications infrastructure

99.999% Uptime ("Five Nines")

Allows approximately 26 seconds of downtime per month. Reserved for:

Emergency services systems
Critical infrastructure
High-frequency trading platforms
Systems where downtime costs millions per minute

SLA vs SLO vs SLI

SLA (Service Level Agreement)

A contractual agreement between service provider and customer that specifies performance expectations. Includes consequences for not meeting targets.

SLO (Service Level Objective)

Internal targets that are typically more strict than SLAs. Provides a buffer before violating customer commitments. Example: 99.95% SLO with 99.9% SLA.

SLI (Service Level Indicator)

Quantitative measure of service performance. The actual metric used to evaluate SLO compliance. Examples: request success rate, response time, uptime percentage.

Cost of Reliability

Each additional "nine" of uptime significantly increases complexity and cost:

99% → 99.9%: Requires redundancy and failover systems
99.9% → 99.99%: Needs multi-region deployment and automated recovery
99.99% → 99.999%: Demands complex distributed systems and 24/7 operations

Calculating Your Target Uptime

When setting uptime targets, consider:

Business Impact: What does downtime cost your business?
User Expectations: What do customers need and expect?
Implementation Cost: What resources are required?
Maintenance Windows: Do you need scheduled maintenance time?
Dependencies: What's the reliability of your dependencies?

Best Practices

Set realistic targets based on current performance
Keep SLOs stricter than SLAs to provide buffer
Measure uptime from the user's perspective
Account for scheduled maintenance in calculations
Monitor continuously and track trends
Review and adjust targets periodically
Use error budgets to balance reliability and innovation

Quick Reference

Annual Downtime

90%	36.5 days
99%	3.65 days
99.9%	8.77 hours
99.99%	52.6 minutes
99.999%	5.26 minutes

Factors Affecting Uptime

Hardware failures
Software bugs and crashes
Network issues
Power outages
Human errors
Security incidents
Planned maintenance
Dependency failures