What Is k6? Complete Guide to k6 Load & Performance Testing

If you are exploring tools for load testing or performance testing, you may have come across k6. But what is k6, what is it used for, and how does it fit into a modern software testing strategy?

k6 is an open-source performance testing tool used to test how applications, APIs, and services behave under different workloads. It allows QA engineers, developers, and performance testers to simulate virtual users, generate traffic, measure application performance, and identify performance issues.

One of the main differences between k6 and many traditional performance-testing tools is that k6 tests are written as code. This makes k6 particularly useful for teams that want performance testing to work alongside source control, automation, and CI/CD pipelines.

In this guide, you'll learn what k6 is, how it works, what it is used for, the types of tests you can perform with it, and when k6 can be useful for your project.

01What Is k6?

k6 is an open-source performance and load testing tool designed to help teams evaluate the reliability and performance of software systems.

With k6, you can create scripts that simulate users interacting with an application or API. You can then control the amount of traffic generated and measure how the system responds.

For example, you could use k6 to determine:

• How an API behaves with 100 concurrent users
• Whether response times increase as traffic grows
• How many requests an application can handle
• Whether an application remains stable during sustained traffic
• What happens when traffic suddenly increases
• Whether an application meets defined performance requirements

k6 is maintained by Grafana and is commonly used as part of modern performance-testing and engineering workflows.

02What Is k6 Used For?

k6 is mainly used for performance and reliability testing.

Common use cases include:

• Load testing
• Stress testing
• Spike testing
• Soak or endurance testing
• API performance testing
• Web performance testing
• Scalability testing
• Performance testing in CI/CD pipelines

For example, suppose an application normally receives 500 requests per second during peak hours.

A QA team could create a k6 test that generates a similar workload and then measure:

• Response time
• Request rate
• Error rate
• Throughput
• Percentile response times

This helps the team understand whether the application can handle its expected workload before real users experience performance problems.

03How Does k6 Work?

A typical k6 test consists of a test script, virtual users, a workload configuration, and performance metrics.

1. Create a test script

You write the actions that you want the simulated users to perform.

For example, an API test might send a GET request:

import http from 'k6/http'; export default function () { http.get('https://example.com/api/products'); }

The script tells k6 what request should be executed.

2. Configure virtual users

k6 uses virtual users (VUs) to simulate users executing your test logic.

For example:

export const options = { vus: 10, duration: '30s', };

This configuration runs the test with 10 virtual users for 30 seconds.

You can also create more advanced workload patterns, such as gradually increasing the number of users instead of starting all users at once.

3. Run the test

Once the script and workload are configured, you run the test with k6.

The tool generates the defined workload and collects performance data.

4. Analyze the results

After the test completes, you can examine metrics such as response times, request rates, errors, and percentiles.

These results help you determine whether the system meets the required performance expectations.

04What Types of Testing Can You Perform With k6?

k6 can support several common performance-testing scenarios.

Load Testing

Load testing evaluates how an application performs under an expected workload.

For example, if an application normally needs to support 1,000 concurrent users, you can create a workload that represents that expected traffic.

The goal is to determine whether the system maintains acceptable performance under normal or anticipated conditions.

Stress Testing

Stress testing pushes the system beyond its expected workload.

You might gradually increase traffic until you observe:

• Increasing response times
• Higher error rates
• Resource exhaustion
• Degraded application performance

This helps identify the system's limits and behavior under excessive load.

Spike Testing

Spike testing evaluates how an application responds to a sudden increase or decrease in traffic.

For example, an e-commerce application might suddenly receive a large number of users during a flash sale.

Soak Testing

Soak testing, also called endurance testing, runs a workload for an extended period.

It can help identify problems that may not appear during a short test, such as:

• Memory leaks
• Resource exhaustion
• Gradually increasing response times
• Long-term stability problems

05What Can You Test With k6?

k6 can be used for different types of applications and services.

APIs

API performance testing is one of the common uses of k6.

You can test:

• REST APIs
• Authentication endpoints
• Search APIs
• Product APIs
• Microservice endpoints
• Other HTTP-based services

For example, you could simulate hundreds of users requesting the same API and measure how the API behaves as traffic increases.

Web Applications

k6 can also be used to evaluate web application performance.

Depending on the testing objective, teams can perform protocol-level testing or use k6's browser capabilities for selected browser-based scenarios.

This distinction is important: generating large amounts of HTTP traffic is different from running a large number of full browser instances.

The appropriate approach depends on what you are trying to measure.

06What Programming Language Does k6 Use?

k6 test scripts use JavaScript, with TypeScript also supported through k6's tooling and workflows.

This makes k6 approachable for teams that already work with JavaScript or TypeScript.

A basic k6 script looks like this:

import http from 'k6/http'; export const options = { vus: 10, duration: '30s', }; export default function () { http.get('https://example.com'); }

You can store these scripts in a source-control system and manage them alongside other project code.

07What Are k6 Metrics?

Performance testing is not simply about checking whether requests succeed. You also need to understand how the system performs under load.

Some important metrics include:

Response time

Measures how long requests take to complete.

Error rate

Shows how frequently requests fail.

Throughput

Shows how much work the system processes during a given period.

Request rate

Shows how frequently requests are generated or processed.

Percentiles

Percentiles provide a better view of response-time distribution than averages alone.

For example:

p50 = 200 ms

p95 = 450 ms

p99 = 800 ms

If the p95 response time is 450 ms, it means 95% of the measured requests completed in 450 ms or less.

This information can help teams understand the experience of slower requests that an average response time might hide.

08What Are k6 Thresholds?

A threshold defines a performance requirement that the test should meet.

For example:

export const options = { thresholds: { http_req_duration: ['p(95)<500'], http_req_failed: ['rate<0.01'], }, };

This example defines two expectations:

• 95% of requests should complete in less than 500 milliseconds.
• The request failure rate should remain below 1%.

Thresholds are useful because they turn performance requirements into measurable criteria.

Instead of saying:

The API should be fast.

You can define a measurable requirement such as:

95% of API requests should complete within 500 milliseconds.

09k6 vs JMeter

k6 and JMeter are both widely used for performance and load testing, but their approaches are different.

There is no single tool that is the right choice for every performance-testing project.

The choice depends on your team's skills, existing technology stack, testing requirements, and automation strategy.

For a more detailed comparison, see QACraft's k6 vs other performance testing tools article.

10Advantages of k6

k6 can be useful for teams that want performance testing to fit into a modern development workflow.

Some advantages include:

• Code-based test creation — Tests can be stored and maintained as code.
• Developer-friendly scripting — JavaScript and TypeScript workflows can be familiar to development and automation teams.
• CI/CD integration — Performance tests can be incorporated into automated pipelines.
• Flexible workloads — Virtual users, scenarios, stages, and durations can be configured according to the test objective.
• API performance testing — k6 is well suited to testing APIs and backend services.
• Performance thresholds — Teams can define measurable performance requirements.

11Limitations of k6

k6 is powerful, but it is not a replacement for performance engineering as a whole.

Some considerations include:

• Teams need to understand the basics of performance testing and workload modeling.
• Code-based testing may require some JavaScript or TypeScript knowledge.
• Browser-based tests can require considerably more resources than protocol-level tests.
• A performance test can show that a system is slow, but additional monitoring may be required to find the root cause.
• Unrealistic test data or workloads can produce misleading results.

A performance-testing tool can generate traffic and collect metrics, but good test design remains the responsibility of the testing team.

12When Should You Use k6?

k6 can be a good option when you need to:

• Test API performance
• Perform load testing
• Perform stress or spike testing
• Run endurance tests
• Validate application scalability
• Automate performance tests
• Add performance checks to CI/CD
• Define measurable performance thresholds
• Maintain performance tests as code

It is particularly useful when QA and development teams want performance testing to become part of the regular software-development lifecycle rather than something performed only before a major release.

13k6 Performance Testing Best Practices

To get useful results from k6, focus on the test strategy as much as the tool.

Define the objective first

Know what you want to learn from the test.

For example:

Can the checkout API handle 500 requests per second while keeping 95% of responses below 500 ms?

That is a measurable performance objective.

Use realistic workloads

Do not select a random number of virtual users.

Base the workload on expected traffic, user behavior, business requirements, and production patterns where available.

Start with a small test

Before running a large load test, make sure the script, authentication, test data, and endpoints work correctly.

Use thresholds

Define measurable criteria so that performance results can be evaluated consistently.

Monitor the application

Do not rely only on k6 metrics.

Combine test results with application, database, infrastructure, and monitoring data when investigating bottlenecks.

Repeat tests consistently

Running comparable tests over time can help identify performance regressions.

14Frequently Asked Questions About k6

What is k6 used for?

k6 is used for performance, load, stress, spike, endurance, API, and reliability testing.

Is k6 a load testing tool?

Yes. k6 can generate controlled workloads to evaluate how applications and APIs behave under load.

Is k6 free?

The open-source k6 tool can be run locally. Grafana also provides cloud-based k6 capabilities.

What language does k6 use?

k6 test scripts use JavaScript, with TypeScript supported through k6's tooling and workflows.

Can k6 test APIs?

Yes. k6 is commonly used for API performance and load testing.

What are virtual users in k6?

Virtual users, or VUs, are simulated users that execute the test script concurrently.

Can k6 be used in CI/CD?

Yes. Because k6 tests can be maintained as code and performance thresholds can be automated, k6 can be integrated into CI/CD workflows.

Is k6 better than JMeter?

Neither tool is universally better. They use different approaches and may suit different teams and testing requirements.

15Conclusion

k6 is an open-source performance testing tool that helps teams understand how applications, APIs, and services behave under load.

Its code-based approach makes it useful for QA engineers, developers, SDETs, and DevOps teams that want performance testing to work alongside automation, version control, and CI/CD.

Whether you are testing an API with hundreds of virtual users, checking application behavior during a traffic spike, or establishing performance thresholds for a deployment pipeline, k6 provides a flexible way to create and automate performance tests.

The most important thing, however, is not simply choosing a performance-testing tool. A useful performance test starts with a clear objective, a realistic workload, measurable performance criteria, and proper analysis of the system under test.

If your application requires structured performance and load testing, QACraft can help you plan and execute performance tests based on your application's architecture, workload, and business requirements.