SLI Calculator
Calculate a Service Level Indicator from good and total events over a measurement window, and compare it against your SLO target.
Inputs
Count of events that met the success criteria (e.g. requests under latency threshold, non-5xx responses).
Total number of valid events measured in the window.
Rolling window over which the SLI is measured (commonly 28 or 30 days).
The Service Level Objective you're measuring against.
SLI (Service Level Indicator)
99.8500%
Meets SLO Target
false
Error Budget Consumed
150.0%
Bad Events
1,500
Margin vs SLO (pp)
-0.0500
Step by step
SLI: good / total × 100
998,500 / 1,000,000 × 100
= 99.8500%
Bad events
1,000,000 − 998,500
= 1,500
Error budget (events allowed to fail)
1,000,000 × (1 − 99.9/100)
= 1000 events
Error budget consumed
1,500 / 1000 × 100
= 150.0%
How it works
An SLI (Service Level Indicator) is the measured proportion of 'good' events out of all valid events in a window — for example, requests served under a latency threshold, or responses that weren't a server error. Formula: SLI = good_events / total_events × 100. Comparing the SLI to your SLO (Service Level Objective) target tells you whether you're in compliance, and the gap between the two — expressed as error budget — tells you how much unreliability you can still afford before breaching the SLO.
Formulas
Service Level Indicator
SLI = (good_events / total_events) × 100
- good_events
- Count of events meeting success criteria
- total_events
- Total valid events in the measurement window
Error budget consumed
error_budget_consumed_% = bad_events / (total_events × (1 - SLO_target / 100)) × 100
- bad_events
- total_events - good_events
- total_events
- Total valid events
- SLO_target
- Service Level Objective target percentage
Frequently Asked Questions
What's the difference between SLI, SLO and SLA?
The SLI is the raw measurement (e.g. 99.95% success rate). The SLO is your internal target for that SLI (e.g. 99.9%). The SLA is the externally communicated commitment, usually with contractual consequences, and is often set looser than the SLO to leave margin.
What counts as a 'good event'?
It depends on the SLI type: for availability, a good event is a non-error response; for latency, it's a request under your threshold; for correctness, it's a response with the expected result. Define it precisely so measurement is consistent.
How is error budget useful day-to-day?
Error budget converts an abstract percentage into a concrete number of allowed failures. Once it's exhausted, many SRE practices pause risky releases and prioritize reliability work until the budget resets in the next window.
Why measure over a rolling window instead of calendar month?
A rolling window (e.g. trailing 28 days) gives a consistent, continuously updated view of reliability without discontinuities at month boundaries, which is why most SRE tooling defaults to it.
You might also need
- SLO Compliance CalculatorCommonly used together
- Monitoring Coverage CalculatorCommonly used together
- Error Budget CalculatorCommonly used together
- Change Failure Rate CalculatorCommonly used together
- Platform Reliability Score CalculatorCommonly used together
- API Gateway Throughput CalculatorCommonly used together