Free Core Web Vitals test

Test Core Web Vitals lab signals without mixing them with field data.

Check synthetic Largest Contentful Paint and Cumulative Layout Shift in a controlled browser, then use TBT, page weight and resource evidence to investigate why the page behaves that way.

Free · No signup required · Real Chromium lab test

What this tool measures

Core Web Vitals signals plus the diagnostics around them

Largest Contentful Paint (LCP)

Measures how long the main visible content takes to render in the current synthetic test.

Cumulative Layout Shift (CLS)

Measures unexpected visual movement observed while this browser run loads the page.

Total Blocking Time (TBT)

A lab diagnostic for long main-thread tasks. SiteState does not relabel TBT as INP.

First Contentful Paint (FCP)

Shows when the first text or image content is painted during the current run.

LCP-critical resources

Highlights resources SiteState associates with the main-content rendering path.

JavaScript impact

Shows sampled CPU and third-party JavaScript impact that can contribute to poor page experience.

Interpret the result

Lab data is best for repeatable debugging.

Core Web Vitals can be discussed using both lab and field measurements, but those datasets are not interchangeable. SiteState's free tool is a synthetic lab test, which makes it useful for controlled debugging and retesting after changes.

Interaction to Next Paint depends on real interaction data. SiteState does not invent an INP value from a page-load-only synthetic test. When field data is available in the future, it should remain a clearly separate layer.

Methodology

This tool reports synthetic lab measurements from a controlled Chromium page load. LCP and CLS are measured in the lab run; TBT is used as a blocking diagnostic. INP is not estimated from the synthetic test.

FAQ

Common questions about this speed test

Can I test Core Web Vitals for free?

Yes. SiteState provides a free browser-based test for synthetic page-experience signals such as LCP and CLS, together with supporting performance diagnostics.

Does this Core Web Vitals test show INP?

No. INP is based on real user interaction data. SiteState does not fabricate an INP value from a synthetic page-load test.

Why can my lab result differ from Google field data?

Lab tests use controlled conditions, while field data reflects real users across different devices, networks, geographies and interactions. Both are useful, but for different purposes.