Every k6 test emits built-in and Custom metrics. Each supported protocol also has its specific metrics.
Standard built-in metrics.
k6 always collects the following metrics, no matter what protocol the test uses:
| Metric Name | Type | Description |
|---|---|---|
| vus | Gauge | Current number of active virtual users |
| vus_max | Gauge | Max possible number of virtual users (VU resources are pre-allocated, to avoid affecting performance when scaling up load ) |
| iterations | Counter | The aggregate number of times the VUs execute the JS script (the default function). |
| iteration_duration | Trend | The time to complete one full iteration, including time spent in setup and teardown. To calculate the duration of the iteration's function for the specific scenario, try this workaround |
| dropped_iterations | Counter | The number of iterations that weren't started due to lack of VUs (for the arrival-rate executors) or lack of time (expired maxDuration in the iteration-based executors). About dropped iterations |
| data_received | Counter | The amount of received data. This example covers how to track data for an individual URL. |
| data_sent | Counter | The amount of data sent. Track data for an individual URL to track data for an individual URL. |
| checks | Rate | The rate of successful checks. |
HTTP-specific built-in metrics
These metrics are generated only when the test makes HTTP requests.
noteFor all http_req_* metrics, the timestamp is emitted the end of the request. In other words, the timestamp happens when k6 receives the end of the response body, or the request times out.
| Metric Name | Type | Description |
|---|---|---|
| http_reqs | Counter | How many total HTTP requests k6 generated. |
| http_req_blocked | Trend | Time spent blocked (waiting for a free TCP connection slot) before initiating the request. float |
| http_req_connecting | Trend | Time spent establishing TCP connection to the remote host. float |
| http_req_tls_handshaking | Trend | Time spent handshaking TLS session with remote host |
| http_req_sending | Trend | Time spent sending data to the remote host. float |
| http_req_waiting | Trend | Time spent waiting for response from remote host (a.k.a. “time to first byte”, or “TTFB”). float |
| http_req_receiving | Trend | Time spent receiving response data from the remote host. float |
| http_req_duration | Trend | Total time for the request. It's equal to http_req_sending + http_req_waiting + http_req_receiving (i.e. how long did the remote server take to process the request and respond, without the initial DNS lookup/connection times). float |
| http_req_failed | Rate | The rate of failed requests according to setResponseCallback. |
Browser metrics
The k6 browser module emits its own metrics based on the Core Web Vitals.
These core metrics will evolve over time when technology changes, but for now, k6 tracks the following core web vitals:
| Core Web Vital | Description |
|---|---|
| browser_web_vital_lcp | Measures a page's loading performance. Please refer to Largest Contentful Paint for more information. |
| browser_web_vital_fid | Measures a page's interactivity. Please refer to First Input Delay for more information. |
| browser_web_vital_cls | Measures a page's visual stability. Please refer to Cumulative Layout Shift for more information. |
Other Web Vitals
Apart from the Core Web Vitals, the browser module also reports Other Web Vitals.
| Other Web Vital | Description |
|---|---|
| browser_web_vital_ttfb | Measures the time it takes between the browser request and the start of the response from a server. Please refer to Time to First Byte for more information. |
| browser_web_vital_fcp | Measures the time it takes for the browser to render the first DOM element on the page, whether that's a text, image or header. Please refer to First Contentful Paint for more information. |
| browser_web_vital_inp | An experimental metric that measures a page's responsiveness. Please refer to Interaction to Next Paint for more information. |
Built-in WebSocket metrics
k6 emits the following metrics when interacting with a WebSocket service through the experimental or legacy websockets API.
| Metric name | Type | Description |
|---|---|---|
| ws_connecting | Trend | Total duration for the WebSocket connection request. |
| ws_session_duration | Trend | Duration of the WebSocket session. Time between the start of the connection and the end of the VU execution. |
| ws_sessions | Counter | Total number of started WebSocket sessions. |
| ws_ping | Trend | Duration between a ping request and its pong reception |
| ws_msgs_sent | Counter | Total number of messages sent |
| ws_msgs_received | Counter | Total number of messages received |
Built-in gRPC metrics
k6 emits the following metrics when it interacts with a service through the gRPC API.
| Metric Name | Type | Description |
|---|---|---|
| grpc_req_duration | Trend | Time to receive response from remote host |
| grpc_streams | Counter | Total number of started streams |
| grpc_streams_msgs_sent | Counter | Total number of messages sent |
| grpc_streams_msgs_received | Counter | Total number of messages received |
noteSteams-related metrics (grpc_streams*) are available only with using k6/experimental/grpc module which is available since k6 version 0.45.0.