SLO Calculator SRE Tool

Calculate Service Level Objectives and error budgets. Determine allowed downtime for your target uptime percentage over a specified period.

Documentation

What is SLO?

Service Level Objectives (SLOs) define the target level of service reliability. They are typically expressed as a percentage of uptime over a time period, representing how much downtime is acceptable.

SLOs help teams balance reliability with development velocity by providing clear targets for system availability.

Key Concepts

  • SLO: Target percentage of time the service should be available
  • Error Budget: Amount of downtime allowed while meeting SLO
  • Burn Rate: How quickly you're consuming your error budget
  • SLA: Contractual agreement based on SLOs

Common SLO Targets

99.9% 8.77h/month 3 nines
99.95% 2.16h/month 3.5 nines
99.99% 4.32m/month 4 nines
99.999% 26s/month 5 nines

Calculate error budgets for different SLO targets and time periods:

99.9% SLO - 30 Days

Common Target
Total Time: 43,200 minutes
Error Budget: 0.1%
Allowed Downtime: 43.2 minutes
Readable: 43m 12s

99.95% SLO - 30 Days

High Availability
Total Time: 43,200 minutes
Error Budget: 0.05%
Allowed Downtime: 21.6 minutes
Readable: 21m 36s

99.99% SLO - 30 Days

Critical Systems
Total Time: 43,200 minutes
Error Budget: 0.01%
Allowed Downtime: 4.32 minutes
Readable: 4m 19s

99.999% SLO - 30 Days

Ultra Reliable
Total Time: 43,200 minutes
Error Budget: 0.001%
Allowed Downtime: 26 seconds
Readable: 0m 26s

Calculation Formula

Allowed Downtime = Total Time × (100% - SLO%) ÷ 100%

Where:

  • Total Time: Time period in minutes (days × 24 × 60)
  • SLO%: Target uptime percentage (e.g., 99.9)
  • 100% - SLO%: Error budget percentage

This SLO Calculator is implemented using pure JavaScript with no external dependencies. It calculates error budgets based on Service Level Objectives using precise mathematical formulas.

Step-by-Step Code Breakdown

1. Input Validation

Validate that SLO is between 0 and 100, and days is positive:

if (slo < 0 || slo > 100 || days <= 0) {
    return '<p>Invalid input values.</p>';
}
// Ensures SLO percentage is valid and time period is positive

2. Total Time Calculation

Calculate total time in minutes for the specified period:

const totalMinutes = days * 24 * 60;
// Converts days to minutes (24 hours/day × 60 minutes/hour)

This gives us the total time window for measuring the SLO.

3. Error Budget Calculation

Calculate allowed downtime in minutes using the SLO formula:

const allowedDowntimeMinutes = totalMinutes * (100 - slo) / 100;
// Error budget = total time × (100% - SLO%) ÷ 100%

This is the core calculation that determines how much downtime is acceptable.

4. Unit Conversions

Convert the result to different time units for better readability:

const allowedDowntimeHours = allowedDowntimeMinutes / 60;
const allowedDowntimeDays = allowedDowntimeHours / 24;
// Convert minutes to hours and days for display

Provides multiple time formats to help users understand the impact.

5. Human-Readable Format

Convert fractional minutes to minutes and seconds:

const minutes = Math.floor(allowedDowntimeMinutes);
const seconds = Math.round((allowedDowntimeMinutes - minutes) * 60);
// Extract whole minutes and remaining seconds

Creates a more intuitive display format (e.g., "43m 12s").

SLO Planning

  • Start realistic: Begin with achievable SLOs (99-99.9%) and improve over time
  • Consider user impact: Different services may need different SLO targets
  • Include error budgets: Plan for incidents and maintenance windows
  • Regular review: Reassess SLOs quarterly based on business needs
  • Document trade-offs: Explain the cost-benefit of higher SLO targets

Monitoring & Alerting

  • Track burn rate: Monitor how quickly you're consuming error budget
  • Set up alerts: Alert when approaching error budget limits
  • Use dashboards: Visualize SLO compliance over time
  • Incident response: Have procedures for when SLOs are at risk
  • Post-mortems: Analyze incidents and their impact on SLOs

Error Budget Management

  • Spend wisely: Use error budget for planned improvements
  • Prevent exhaustion: Slow down releases when budget is low
  • Team autonomy: Give teams control over their error budgets
  • Graduated responses: Different actions at 50%, 80%, 100% budget used
  • Reset periods: Define when error budgets reset (monthly/quarterly)

Practical Applications

Capacity Planning

Determine realistic uptime targets for infrastructure investments and scaling decisions.

Incident Response

Know how much downtime you can afford before violating service level agreements.

Team Communication

Set clear expectations with stakeholders about service reliability and trade-offs.

Cost Optimization

Balance reliability costs with business value and user experience requirements.