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Metrics measure how a system performs under test conditions. By default, k6 automatically collects built-in metrics. Besides built-ins, you can also make custom metrics.

Metrics fall into four broad types:

  • Counters sum values.
  • Gauges track the smallest, largest, and latest values.
  • Rates track how frequently a non-zero value occurs.
  • Trends calculate statistics for multiple values (like mean or mode).

Built-in metrics

The built-in metrics output to stdout when you run the simplest possible k6 test:

1import http from 'k6/http';
3export default function () {
4 http.get('');

Running the preceding script outputs something like this:

$ k6 run script.js
/\ |‾‾| /‾‾/ /‾‾/
/\ / \ | |/ / / /
/ \/ \ | ( / ‾‾\
/ \ | |\ \ | () |
/ __________ \ |__| \__\ \_____/ .io
execution: local
script: http_get.js
output: -
scenarios: (100.00%) 1 scenario, 1 max VUs, 10m30s max duration (incl. graceful stop):
* default: 1 iterations for each of 1 VUs (maxDuration: 10m0s, gracefulStop: 30s)
running (00m03.8s), 0/1 VUs, 1 complete and 0 interrupted iterations
default ✓ [======================================] 1 VUs 00m03.8s/10m0s 1/1 iters, 1 per VU
data_received..................: 22 kB 5.7 kB/s
data_sent......................: 742 B 198 B/s
http_req_blocked...............: avg=1.05s min=1.05s med=1.05s max=1.05s p(90)=1.05s p(95)=1.05s
http_req_connecting............: avg=334.26ms min=334.26ms med=334.26ms max=334.26ms p(90)=334.26ms p(95)=334.26ms
http_req_duration..............: avg=2.7s min=2.7s med=2.7s max=2.7s p(90)=2.7s p(95)=2.7s
{ expected_response:true }...: avg=2.7s min=2.7s med=2.7s max=2.7s p(90)=2.7s p(95)=2.7s
http_req_failed................: 0.00% ✓ 01
http_req_receiving.............: avg=112.41µs min=112.41µs med=112.41µs max=112.41µs p(90)=112.41µs p(95)=112.41µs
http_req_sending...............: avg=294.48µs min=294.48µs med=294.48µs max=294.48µs p(90)=294.48µs p(95)=294.48µs
http_req_tls_handshaking.......: avg=700.6ms min=700.6ms med=700.6ms max=700.6ms p(90)=700.6ms p(95)=700.6ms
http_req_waiting...............: avg=2.7s min=2.7s med=2.7s max=2.7s p(90)=2.7s p(95)=2.7s
http_reqs......................: 1 0.266167/s
iteration_duration.............: avg=3.75s min=3.75s med=3.75s max=3.75s p(90)=3.75s p(95)=3.75s
iterations.....................: 1 0.266167/s
vus............................: 1 min=1 max=1
vus_max........................: 1 min=1 max=1

In that output, all the metrics that start with http, iteration, and vu are built-in metrics, which get written to stdout at the end of a test.

k6 always collects the following built-in metrics:

Metric NameTypeDescription
vusGaugeCurrent number of active virtual users
vus_maxGaugeMax possible number of virtual users (VU resources are pre-allocated, ensuring performance will not be affected when scaling up the load level)
iterationsCounterThe aggregate number of times the VUs executed the JS script (the default function).
iteration_durationTrendThe time it took 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.

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.

Accessing HTTP timings from a script

To access the timing information from an individual HTTP request, the Response.timings object provides the time spent on the various phases in ms:

  • blocked: equals to http_req_blocked.
  • connecting: equals to http_req_connecting.
  • tls_handshaking: equals to http_req_tls_handshaking.
  • sending: equals to http_req_sending.
  • waiting: equals to http_req_waiting.
  • receiving: equals to http_req_receiving.
  • duration: equals to http_req_duration.
1import http from 'k6/http';
3export default function () {
4 const res = http.get('');
5 console.log('Response time was ' + String(res.timings.duration) + ' ms');

The expected (partial) output looks like this:

$ k6 run script.js
INFO[0001] Response time was 337.962473 ms source=console

Custom metrics

You can also create custom metrics. They are reported at the end of a load test, just like HTTP timings:

1import http from 'k6/http';
2import { Trend } from 'k6/metrics';
4const myTrend = new Trend('waiting_time');
6export default function () {
7 const r = http.get('');
8 myTrend.add(r.timings.waiting);
9 console.log(; // waiting_time

The preceding code creates a Trend metric called waiting_time. In the code, it's referred to with the variable name myTrend.

Custom metrics are reported at the end of a test. Here's how the output might look:

$ k6 run script.js
INFO[0001] waiting_time source=console
iteration_duration.............: avg=1.15s min=1.15s med=1.15s max=1.15s p(90)=1.15s p(95)=1.15s
iterations.....................: 1 0.864973/s
waiting_time...................: avg=265.245396 min=265.245396 med=265.245396 max=265.245396 p(90)=265.245396 p(95)=265.245396

You can optionally tag any value for a custom metric. This can be useful when analyzing test results.


Custom metrics are collected from VU threads only at the end of a VU iteration. For long-running scripts, custom metrics might appear only after the test runs a while.

Metric types

All metrics (both built-in and custom) have a type. The four different metric types in k6 are:

Metric typeDescription
CounterA metric that cumulatively sums added values.
GaugeA metric that stores the min, max and last values added to it.
RateA metric that tracks the percentage of added values that are non-zero.
TrendA metric that allows for calculating statistics on the added values (min, max, average and percentiles).

Counter (cumulative metric)

1import { Counter } from 'k6/metrics';
3const myCounter = new Counter('my_counter');
5export default function () {
6 myCounter.add(1);
7 myCounter.add(2);

The preceding code generates something like the following output:

$ k6 run script.js
iteration_duration...: avg=16.48µs min=16.48µs med=16.48µs max=16.48µs p(90)=16.48µs p(95)=16.48µs
iterations...........: 1 1327.67919/s
my_counter...........: 3 3983.037571/s

If you run the script for one iterationwithout specifying --iterations or --durationthe value of my_counter will be three.

Note that there is currently no way to access the value of any custom metric from within JavaScript. Note also that counters that have a value of zero (0) at the end of a test are a special case. They will NOT print to the stdout summary.

Gauge (keep the latest value only)

1import { Gauge } from 'k6/metrics';
3const myGauge = new Gauge('my_gauge');
5export default function () {
6 myGauge.add(3);
7 myGauge.add(1);
8 myGauge.add(2);

The preceding code results in output like this:

$ k6 run script.js
iteration_duration...: avg=21.74µs min=21.74µs med=21.74µs max=21.74µs p(90)=21.74µs p(95)=21.74µs
iterations...........: 1 1293.475322/s
my_gauge.............: 2 min=1 max=3

The value of my_gauge will be 2 at the end of the test. As with the Counter metric, a Gauge with a value of zero (0) will NOT be printed to the stdout summary at the end of the test.

Trend (collect trend statistics (min/max/avg/percentiles) for a series of values)

1import { Trend } from 'k6/metrics';
3const myTrend = new Trend('my_trend');
5export default function () {
6 myTrend.add(1);
7 myTrend.add(2);

The preceding code outputs something like this:

$ k6 run script.js
iteration_duration...: avg=20.78µs min=20.78µs med=20.78µs max=20.78µs p(90)=20.78µs p(95)=20.78µs
iterations...........: 1 1217.544821/s
my_trend.............: avg=1.5 min=1 med=1.5 max=2 p(90)=1.9 p(95)=1.95

A trend metric holds a set of sample values, which it can output statistics about (min, max, average, median, or percentiles). By default, k6 prints average, min, max, median, 90th percentile, and 95th percentile.

Rate (keeps track of the percentage of values in a series that are non-zero)

1import { Rate } from 'k6/metrics';
3const myRate = new Rate('my_rate');
5export default function () {
6 myRate.add(true);
7 myRate.add(false);
8 myRate.add(1);
9 myRate.add(0);

The preceding code outputs something like this:

$ k6 run script.js
iteration_duration...: avg=22.12µs min=22.12µs med=22.12µs max=22.12µs p(90)=22.12µs p(95)=22.12µs
iterations...........: 1 1384.362792/s
my_rate..............: 50.00% ✓ 22

The value of my_rate at the end of the test will be 50%, indicating that half of the values added to the metric were non-zero.

Metric graphs in k6 Cloud Results

If you use k6 Cloud Results, you can access all test metrics within the Analysis Tab. You can use this tab to analyze, compare, and look for meaningful correlations in your test result data.

k6 Cloud Analysis Tab