Understanding Error Rate
How we calculate. Error % = (failures ÷ attempts) × 100. The form uses the same arithmetic as the worked examples on this page. See our methodology and accuracy policy.
Real-world scenario: A typical Error Rate case uses failed requests 750 and total requests 50000. Enter the same figures below to reproduce the worked path.
What is Error Rate?
A primary reliability signal in performance tests. Define “failure” clearly (HTTP 5xx, timeouts, assertion fails).
- Align failure taxonomy across tools
- Separate client vs server errors when diagnosing
- Pair with throughput — low errors at tiny load can mislead
The Formula
Worked Example
Common Use Cases
- SLA gates: max error % for go-live
- Soak tests: watch error drift over hours
- Incident reviews: quantify blast radius
Pro Tips
- Timeouts count as errors if users experience failure
- Do not hide retries that mask true fail rates
- Break down by endpoint
Limitations: Error Rate figures are educational performance-testing aids—not contractual capacity guarantees. Validate with calibrated load tools and production telemetry.
FAQ
Are 4xx errors failures?
Depends on the test: expected 401/404 checks may be successes; unexpected 4xx usually count as fails.
Availability vs error rate?
They are related but not identical—availability often weights downtime windows; error rate weights request outcomes.
Authoritative References
For performance and quality engineering context, consult:
- ISTQB — performance testing syllabus topics
- W3C Navigation Timing — browser performance timing
- CMG / capacity forums — capacity and performance practice communities