What is the difference between SLA, SLO, and SLI?

SLA (Service Level Agreement) is a contract with customers. SLO (Service Level Objective) is an internal target for service reliability. SLI (Service Level Indicator) is the actual measured metric like uptime or latency. SLIs inform whether you are meeting SLOs, which determine SLA compliance.

SLA Calculator - Uptime & Error Budget Tool

What Is SLA/SLO Calculation

Service Level Agreements (SLAs) and Service Level Objectives (SLOs) define the expected reliability and performance of services in quantitative terms. An SLA is a contractual commitment (with financial penalties for breaches), while an SLO is an internal target that teams use to balance reliability investment against feature development.

Understanding the mathematics behind availability percentages, error budgets, and downtime calculations is essential for platform engineering, DevOps, SRE (Site Reliability Engineering), and service management.

Availability and Downtime

Availability	Annual Downtime	Monthly Downtime	Called
99%	3.65 days	7.3 hours	"Two nines"
99.9%	8.77 hours	43.8 minutes	"Three nines"
99.95%	4.38 hours	21.9 minutes	Common SaaS SLA
99.99%	52.6 minutes	4.38 minutes	"Four nines"
99.999%	5.26 minutes	26.3 seconds	"Five nines"

Error Budget

Error budget is the inverse of SLO — the amount of unreliability your service can tolerate before violating its objective:

Error Budget = 1 - SLO

With a 99.9% SLO, your error budget is 0.1% — approximately 43 minutes of downtime per month. When the error budget is consumed, teams should freeze deployments and focus on reliability.

Key Metrics

Metric	Definition	Example Target
Availability	Percentage of time the service is operational	99.95%
Latency (p50)	Median response time	< 100ms
Latency (p99)	99th percentile response time	< 500ms
Error rate	Percentage of requests that fail	< 0.1%
Throughput	Requests processed per second	> 10,000 rps

Common Use Cases

SLO definition: Calculate appropriate availability targets based on business requirements and the cost of additional reliability engineering
Error budget tracking: Compute remaining error budget to decide whether to prioritize new features or reliability improvements
SLA negotiation: Understand the operational cost of different availability commitments before agreeing to contractual SLAs
Incident impact assessment: Calculate the percentage of error budget consumed by each incident to prioritize post-incident improvements
Capacity planning: Determine how much redundancy is needed to achieve target availability levels

Best Practices

Set SLOs slightly above SLAs — Your internal target (SLO) should be stricter than your contractual commitment (SLA). This provides a buffer before SLA penalties are triggered.
Use error budgets to make decisions — When error budget is healthy, ship features faster. When it's consumed, invest in reliability. This creates a data-driven balance between velocity and stability.
Measure from the user's perspective — Measure availability at the edge (load balancer, CDN) rather than at the server. Users don't care if your servers are up if the network path is down.
Define SLIs clearly — Service Level Indicators (the metrics backing your SLOs) must be precisely defined. "Availability" can mean many things — specify exactly what counts as an error.
Each additional nine costs 10x — Moving from 99.9% to 99.99% typically requires 10x the engineering investment. Ensure the business value justifies the cost before committing to higher targets.

SLA/SLO Calculator