Core Web Vitals (CrUX) checker

What are Core Web Vitals?

Core Web Vitals are three metrics Google uses to measure how a page feels to real visitors: how fast the main content appears, how quickly the page responds, and how stable the layout is.

• Largest Contentful Paint (LCP): how long until the largest image or text block in the viewport is shown. It stands for loading speed.

• Interaction to Next Paint (INP): how long the page takes to show a response after a click, tap or key press. It replaced First Input Delay (FID) in March 2024.

• Cumulative Layout Shift (CLS): how much visible content moves around unexpectedly while the page is open.

Two more metrics help explain them. First Contentful Paint (FCP) is when the first text or image appears. Time to First Byte (TTFB) is how long the server takes to start sending the page. Neither is a Core Web Vital, but a slow TTFB or FCP almost always means a slow LCP.

Core Web Vitals thresholds

Each metric is rated at the 75th percentile of visits, separately for phones and desktops. These are the thresholds Google publishes:

Sources: web.dev, Defining the Core Web Vitals thresholds · FCP · TTFB. Google calls the TTFB thresholds a rough guide.

What is CrUX data?

The Chrome UX Report (CrUX) is Google's public dataset of how real Chrome users experience websites. It is the same field data shown in PageSpeed Insights and in the Core Web Vitals report in Google Search Console.

CrUX includes visits from Chrome on desktop and on Android from users who have opted in to sharing usage statistics. It does not include Chrome on iOS, other Chromium browsers such as Edge, or apps that open pages in a WebView.

Each value covers the previous 28 days and is reported at the 75th percentile: three out of four visits were at least this fast. That makes CrUX a measure of what most visitors actually got, not what one test run on a fast machine saw.

Field data vs lab data

Lab tools like Lighthouse load a page once under fixed, simulated conditions. They are good for debugging because you can repeat them. Field data like CrUX averages thousands of real visits across real devices and networks. When the two disagree, field data shows what visitors experience, and lab data helps you find out why.

How this Core Web Vitals checker works

Enter a page URL or switch to the whole origin. The checker asks the CrUX API for the latest 28-day data for all devices and for each device type, and the CrUX History API for up to 40 weekly data points. It shows the 75th percentile and the share of good, needs improvement and poor visits for each metric.

The checker does not load your page or run a test. Everything comes from visits Chrome users have already made, so results are the same no matter where you run it from.

How to read the results

The Core Web Vitals assessment

The assessment passes when LCP, INP and CLS are all good at the 75th percentile. One metric in needs improvement or poor fails it. FCP and TTFB don't count toward it. This is the same rule PageSpeed Insights uses.

p75 value and distribution

The large number is the 75th percentile. The bar under it shows how all visits split between good, needs improvement and poor. Two pages with the same p75 can differ a lot: one may have a long tail of very slow visits.

Devices

Phones are usually slower than desktops because of weaker processors and networks. Google evaluates them separately, so check the phone tab even when the overall result passes.

URL vs origin

URL data covers one page. Origin data covers every page on the domain together, for example everything under https://example.com. Use URL data to judge a specific template, and origin data when a page has too little traffic or to see the whole site.

History

Each point on the chart is one 28-day window, one per week. Because windows overlap, a change on your site appears as a slope over about four weeks rather than a jump.

Why does my site have no CrUX data?

CrUX only includes pages and origins that are public and get enough Chrome visits. Google doesn't publish the exact traffic threshold. Common reasons for missing data:

• The page doesn't get enough visits from opted-in Chrome users in 28 days.

• The page is not indexable: it returns a non-200 status or has a noindex tag or header.

• The page or site is new, and the first 28-day window isn't complete yet.

• One device type has data and another doesn't, because each is counted separately.

Try the whole origin first. If it has no data either, use lab tests to measure the page. Check your page size.

How to improve Core Web Vitals

Improve LCP

• Find the LCP element in DevTools and make sure it isn't lazy-loaded.

• Add fetchpriority="high" to the LCP image and serve it in WebP or AVIF at the displayed size.

• Reduce render-blocking CSS and JavaScript in the head.

• Fix a slow TTFB first: LCP can't start before the HTML arrives.

Improve INP

• Break long JavaScript tasks into smaller ones so the browser can respond in between.

• Remove or delay third-party scripts that run on every interaction.

• Keep the DOM small and avoid re-rendering large parts of the page on each click.

Improve CLS

• Set width and height, or aspect-ratio, on images, videos and embeds.

• Reserve space for ads, banners and cookie notices before they load.

• Use font-display and matched fallback fonts so text doesn't jump when web fonts load.

Improve TTFB and FCP

• Cache HTML at a CDN close to your visitors where the content allows it.

• Cut redirects, especially chains like http → https → www.

• Profile slow database queries and server-side rendering.

• Compress HTML, CSS and JavaScript. Check gzip and Brotli.

Why your scores haven't changed after a fix

Every CrUX value is an average of the previous 28 days. The day after you ship a fix, 27 of those days are still from before it. The new value moves a little each day and fully reflects the change after about four weeks.

The CrUX API refreshes daily. The History API adds one new weekly point every Monday. To confirm a fix sooner, measure it with lab tests or your own real-user monitoring and use CrUX to confirm the trend.

Do Core Web Vitals affect SEO?

Yes, but as one signal among many. Google says its ranking systems use Core Web Vitals as part of page experience, and that the data comes from CrUX. It also says good page experience doesn't replace relevant, helpful content.

In practice Core Web Vitals help most when competing pages are similarly relevant. A failing assessment is also a sign that visitors are leaving, which matters with or without rankings.