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Built-in metrics

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 NameTypeDescription
vusGaugeCurrent number of active virtual users
vus_maxGaugeMax possible number of virtual users (VU resources are pre-allocated, to avoid affecting performance when scaling up load )
iterationsCounterThe aggregate number of times the VUs execute the JS script (the default function).
iteration_durationTrendThe 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_iterationsCounterThe 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_receivedCounterThe amount of received data. This example covers how to track data for an individual URL.
data_sentCounterThe amount of data sent. Track data for an individual URL to track data for an individual URL.
checksRateThe rate of successful checks.

HTTP-specific built-in metrics

These metrics are generated only when the test makes HTTP requests.


For 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 NameTypeDescription
http_reqsCounterHow many total HTTP requests k6 generated.
http_req_blockedTrendTime spent blocked (waiting for a free TCP connection slot) before initiating the request. float
http_req_connectingTrendTime spent establishing TCP connection to the remote host. float
http_req_tls_handshakingTrendTime spent handshaking TLS session with remote host
http_req_sendingTrendTime spent sending data to the remote host. float
http_req_waitingTrendTime spent waiting for response from remote host (a.k.a. “time to first byte”, or “TTFB”). float
http_req_receivingTrendTime spent receiving response data from the remote host. float
http_req_durationTrendTotal 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_failedRateThe 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 VitalDescription
webvital_largest_content_paintMeasures a page's loading performance. Please refer to Largest Contentful Paint for more information.
webvital_first_input_delayMeasures a page's interactivity. Please refer to First Input Delay for more information.
webvital_cumulative_layout_shiftMeasures 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 VitalDescription
webvital_time_to_first_byteMeasures 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.
webvital_first_contentful_paintMeasures 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.
webvital_interaction_to_next_paintAn experimental metric that measures a page's responsiveness. Please refer to Interaction to Next Paint for more information.

Legacy browser metrics


As of k6 version 0.44.0, k6 now natively supports Google's Core Web Vitals. In future releases, to align with recommended practices, we will remove the following browser metrics to focus on the web vital metrics. Please refer to #831 for more information.

Apart from the usual HTTP specific metrics that k6 already tracks, the browser module tracks the following browser specific performance metrics on top:

Metric NameDescription
browser_dom_content_loadedEmitted when the page is loaded but scripts have just started to be executed.
browser_first_paintEmitted when the browser renders the first pixel on the page.
browser_loadedEmitted when the page is fully loaded.

Built-in WebSocket metrics

k6 emits the following metrics when interacting with a WebSocket service through the experimental or legacy websockets API.

Metric nameTypeDescription
ws_connectingTrendTotal duration for the WebSocket connection request.
ws_session_durationTrendDuration of the WebSocket session. Time between the start of the connection and the end of the VU execution.
ws_sessionsCounterTotal number of started WebSocket sessions.
ws_pingTrendDuration between a ping request and its pong reception
ws_msgs_sentCounterTotal number of messages sent
ws_msgs_receivedCounterTotal number of messages received

Built-in gRPC metrics

k6 emits the following metrics when it interacts with a service through the gRPC API.

Metric NameTypeDescription
grpc_req_durationTrendTime to receive response from remote host