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APIs · head to head

curl vs k6

curl logo

curl

APIs

Command-line tool for transferring data using URLs with wide protocol support

From
Free
Rated
-
Full reviewWebsite
k6 logo

k6

Testing

Load testing tool with JavaScript tests on a Go runtime, now owned by Grafana Labs

From
Free
Rated
-
Full reviewWebsite

The short version

  • Each has a real cost: curl open-source with no vendor pricing: curl is free and open-source under GPL 3.0; no subscription or licensing costs apply; k6 tests run on a JavaScript engine embedded in Go rather than on Node.js, so npm packages, Node built-ins and existing shared test utilities generally do not work without being bundled to plain JavaScript, and often not even then.
  • They diverge on capability: curl covers HTTP/HTTPS, k6 covers JavaScript test scripts.
  • Prices and features above were last checked on 30 August 2026.

Where they differ

Only the attributes on which curl and k6 actually diverge.

Attributes where curl and k6 differ
Attributecurlk6
Pricing modelopen-sourceOpen source, no licence fee
PlatformsLinux, MacOS, Windows, Unix, MobileWindows, macOS, Linux, Docker, Kubernetes
CategoryAPIsTesting
Founded1997Unknown

Identical on both: starting price (Free), free tier (Yes), user rating (Not yet rated).

What each one covers

Drawn from each product's published feature list. An absence here means we hold no record of it - not that the product lacks it.

Only in curl

  • HTTP/HTTPS
  • FTP
  • Multiple protocols
  • Universal compatibility
  • Scripting tools
  • Linux support
  • MacOS support
  • Windows support

Only in k6

  • JavaScript test scripts
  • Thresholds and checks
  • Protocol coverage
  • Browser module
  • Executors
  • xk6 extensions
  • Metrics output
  • Kubernetes operator

What people use each for

The jobs each tool is most often brought in to do.

curl

  • API Developmentnot k6
  • API Gatewaynot k6
  • API Testingnot k6
  • API Documentationnot k6
  • Microservicesnot k6

k6

  • Gating a deployment pipeline on a threshold such as 95th percentile latency under a defined arrival ratenot curl
  • Generating tens of thousands of requests per second from a small number of machines without a load generator fleetnot curl
  • Testing gRPC and WebSocket services with the same tooling and thresholds used for HTTP endpointsnot curl
  • Correlating a load test against existing dashboards for an organisation already running Grafana and Prometheusnot curl

Where each one falls short

Documented limitations, not opinions. Every one is a constraint you would hit in normal use.

curl

  • Open-source with no vendor pricing: curl is free and open-source under GPL 3.0; no subscription or licensing costs apply
  • Commercial support costs hidden: site references 'commercial support available' but pricing is not detailed or accessible from homepage
  • No SaaS model: curl is a command-line tool and library; costs are limited to optional commercial support contracts (pricing unknown)
  • Self-support burden: free software means customers rely on community forums or undisclosed commercial support pricing

k6

  • Tests run on a JavaScript engine embedded in Go rather than on Node.js, so npm packages, Node built-ins and existing shared test utilities generally do not work without being bundled to plain JavaScript, and often not even then.
  • The open source binary runs on a single machine, so distributed load requires the Kubernetes operator or the cloud service, and the operator is infrastructure your team has to run and debug.
  • The AGPL-3.0 licence on the core requires legal review at any company that intends to embed k6 in a product it distributes or offers as a service, which is a slower path than a permissively licensed tool.
  • There is no result storage in the open source tool, so trend history means standing up Prometheus, InfluxDB or Grafana Cloud before load testing produces anything comparable over time.
  • Since the Grafana Labs acquisition the roadmap serves the Grafana observability stack, so a buyer choosing k6 as a standalone tool should expect the standalone experience to receive less investment than the cloud and dashboard integration.
  • Grafana Cloud billing is by virtual user hour, so a long soak test at modest concurrency costs far more than its peak load suggests, and cost planning has to be done in virtual user hours rather than in test runs.

Pricing, plan by plan

curl

Free
  • Open SourceFree
    • Full curl functionality
    • Community support

k6

Free
  • k6 OSSFree
    • AGPL-3.0 licence
    • Full test engine, no virtual user limits
    • Runs on one machine unless distributed yourself
  • Grafana Cloud free tierFree
    • Includes a monthly allowance of virtual user hours
    • Hosted test results and dashboards
    • Shared with the rest of the Grafana Cloud free tier
  • Grafana Cloud k6$undefined/month
    • Billed by virtual user hours consumed
    • Load generation from multiple regions
    • Retained test history and comparison

Which should you pick?

Choose curl if

  • You need http/https.
  • You want to start without paying.
  • You work on Linux, MacOS, Windows, Unix, Mobile.
  • You also want ftp.
curl review

Choose k6 if

  • You need javascript test scripts.
  • You want to start without paying.
  • You work on Windows, macOS, Linux, Docker, Kubernetes.
  • You also want thresholds and checks.
k6 review

Questions people ask

Is curl or k6 better?
Neither clearly leads. curl starts at Free and k6 at Free, and user ratings are close enough to be indistinguishable. Choose on capability and platform support.
Which is cheaper, curl or k6?
curl starts at Free and k6 at Free.
Does curl or k6 run on more platforms?
curl runs on Linux, MacOS, Windows, Unix, Mobile. k6 runs on Windows, macOS, Linux, Docker, Kubernetes.
Can I use curl for free?
Both have a free tier, so you can try either at no cost before committing.
What is curl best used for?
curl is most often used for api development, api gateway, api testing, api documentation. Of those, api development and api gateway are not what k6 is typically brought in for.
What can curl do that k6 cannot?
curl covers HTTP/HTTPS, FTP, Multiple protocols, Universal compatibility. k6 covers JavaScript test scripts, Thresholds and checks, Protocol coverage, Browser module.

Answered from the vendors’ own pages

curl: How much does curl cost?

curl is free and open-source software under the GPL 3.0 license. There is no cost to download, use, or modify the software.

Source
k6: Can I use npm packages in a k6 test?

Usually not. Scripts run on a Go-embedded JavaScript engine with no Node.js runtime, so packages depending on Node built-ins fail and pure JavaScript libraries must be bundled first.

curl: Is commercial support available for curl?

Yes. The curl website references 'commercial support available,' but pricing and details are not disclosed on the homepage and require visiting the support page.

Source
k6: Is k6 free?

The core tool is free under AGPL-3.0 with no virtual user limits. Grafana Cloud k6 is a paid service billed by virtual user hours, with a free monthly allowance.

curl: Can I sponsor curl financially?

Yes. The curl website offers an option to 'Sponsor curl financially.' Visitors can donate to support curl development, but the donation link must be accessed through the site's support section.

Source
k6: Does the Grafana acquisition affect the open source tool?

The tool remains open source and maintained, but investment is directed at making it a data source for Grafana, so standalone use is supported rather than prioritised.

k6: Can k6 replace a browser testing tool?

No. The browser module measures page-level metrics under load, but functional browser testing belongs in Playwright or a similar tool.

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