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Core Web Vitals
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The most thorough core web vitals test available — free. Run a full CWV checker on any URL: check LCP, INP, CLS, TTFB, render-blocking resources, HTTP compression, browser caching, image optimisation, font loading, and 20+ signals Google uses to assess Core Web Vitals and Page Experience rankings. No login. Instant results.

LCP, INP & CLS signal analysis
TTFB measured server-side
HTTP compression & cache headers
Actionable fix for every issue
🖼️
LCP Analysis
Load time, hero preload, compression
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CLS Risk Check
Image dims, iframes, ad injections
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INP / JS Audit
Sync scripts blocking main thread
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Server Headers
GZIP, Brotli, Cache-Control, HTTP/2
How the CWV Test Works

How This Core Web Vitals Checker Works

Our core web vitals test fetches your page server-side, measures real TTFB, and analyses 20+ heuristic signals mapped directly to LCP, INP, CLS, and Google's Page Experience assessment criteria.

Enter URL

Paste any URL — homepage, blog post, product page. Our core web vitals checker works on any publicly accessible URL.

Fetch & Measure

We fetch your page using Chrome 120 desktop UA, measuring real server TTFB, response headers, HTML size, and HTTP protocol.

20+ CWV Checks

We run 20+ checks across LCP signals, CLS risk factors, INP/JS analysis, compression, caching, font loading, and resource hints.

Metric Cards

Each Core Web Vital — LCP, INP, CLS, TTFB — gets a Good / Needs Improvement / Poor status card with a measurement value.

Actionable Fixes

Every issue includes the exact fix — specific HTML, CSS, or server config change needed to improve your Core Web Vitals score.

Core Web Vitals Guide

What Google Measures with Core Web Vitals

Google's Core Web Vitals are the three user-experience metrics that form the technical backbone of the Page Experience ranking signal. Here's what each metric measures and how to pass the test.

LCP (Largest Contentful Paint): The Most Important Core Web Vitals Metric

LCP measures when the largest visible element in the viewport — typically a hero image, large heading, or background image — finishes rendering. It is the most heavily weighted Core Web Vitals metric for SEO because it directly reflects perceived load speed. Good LCP: under 2.5 seconds. Needs Improvement: 2.5s–4s. Poor: over 4s.

The top LCP fixes, ranked by impact: (1) Preload your hero image — add <link rel="preload" as="image" href="hero.webp"> to start downloading before the browser reaches the img tag. (2) Convert to WebP or AVIF — 25–50% smaller than JPEG/PNG with no visible quality loss. (3) Enable GZIP/Brotli compression — reduces HTML, CSS, JS transfer size by 60–80%. (4) Use a CDN — serves assets from the nearest edge node, reducing TTFB. (5) Eliminate render-blocking CSS/JS — every millisecond saved before first paint is a millisecond saved on LCP. Use our Page Speed Checklist Tool for a complete LCP audit.

CLS (Cumulative Layout Shift): Stop Your Page Moving Around

CLS measures how much the page layout unexpectedly shifts during loading. Every time an element moves — because an image loaded, an ad was injected, or a font swapped — it contributes to your CLS score. Good CLS: under 0.1. Needs Improvement: 0.1–0.25. Poor: over 0.25. A CLS of 0.1 means visible elements shifted by 10% of the viewport area.

The causes and fixes: Images without width/height — the browser reserves no space, layout shifts when the image loads. Fix: always set explicit width and height on every img tag. Iframes and ads without reserved space — use min-height on ad containers so the layout doesn't reflow when the ad loads. Web fonts causing text reflow (FOIT/FOUT) — use font-display:swap and preload your fonts. Dynamically injected content — avoid inserting banners, cookie notices, or content above existing page elements. Check your mobile-friendly test to see how CLS affects mobile users specifically.

INP (Interaction to Next Paint): Google's Newest Core Web Vitals Metric

INP replaced FID (First Input Delay) as a Core Web Vitals metric in March 2024. While FID only measured the delay before the browser started processing the first interaction, INP measures the complete delay from any user interaction (click, tap, or keystroke) to the next visual update throughout the page's lifetime — not just the first interaction. Good INP: under 200ms. Needs Improvement: 200ms–500ms. Poor: over 500ms.

INP is caused by long JavaScript tasks that monopolise the main thread. When the main thread is busy executing JS, it cannot process user interactions. The primary fixes: add async or defer to all non-critical scripts, code-split large JavaScript bundles (load only what's needed for the current page), use requestIdleCallback for non-urgent work, and avoid long-running event handlers. Use the Chrome DevTools Performance panel to identify Long Tasks (any task over 50ms) in your JS. Also check your JavaScript Dependency Detector for render-blocking script patterns.

TTFB, Compression & Caching: The Foundation of Good Core Web Vitals

TTFB (Time to First Byte) is not itself a Core Web Vitals metric, but it is the prerequisite to all of them. The browser cannot start parsing HTML, discovering resources, or rendering anything until the first byte arrives. A slow TTFB cascades directly into poor LCP and FCP. Good TTFB: under 200ms. Needs Improvement: 200ms–800ms. Poor: over 800ms.

The three server-level quick wins that dramatically improve TTFB and LCP: (1) GZIP or Brotli compression — reduces HTML/CSS/JS transfer by 60–80%. Add to your .htaccess or nginx config. If this check fails, it is the single most impactful free fix available. (2) Cache-Control headers — browser caching means repeat visitors load the page from local cache, with near-zero TTFB. Set Cache-Control: max-age=31536000 for versioned static assets. (3) HTTP/2 or HTTP/3 — multiplexed requests load multiple resources in parallel over one connection, reducing the impact of render-blocking resources. Check your Webpage Size Checker for total resource weight analysis.

Core Web Vitals, AI Search & LLM Discoverability (AEO)

In 2026, passing a core web vitals test isn't just about Google Search rankings — it directly affects how AI search engines parse and cite your content. Google AI Overviews, Perplexity AI, ChatGPT browsing, Claude, and Gemini all crawl web pages to build responses. These AI crawlers behave similarly to Googlebot: they request HTML, parse structured content, and evaluate technical quality signals.

Pages with slow TTFB, render-blocking resources, or large uncompressed HTML are more likely to be partially fetched or skipped by AI crawlers with time limits. Fast, technically optimised pages are more completely and accurately indexed by both Google and AI systems. For AEO (Answer Engine Optimisation), combine good Core Web Vitals with FAQ and HowTo Schema — these structured formats are directly consumed by Google AI Overviews and surface your content as cited sources in AI-generated answers. Also ensure your robots.txt allows GPTBot, ClaudeBot, PerplexityBot, and Googlebot-Extended to crawl your pages.

Lab Data vs Field Data: Understanding CWV Measurement

There are two types of Core Web Vitals data: Lab data (synthetic measurements from tools like Lighthouse, PageSpeed Insights, and this checker) simulates a page load under controlled conditions using defined network speeds and device emulation. It is reproducible and useful for debugging. Field data (real-user measurements from the Chrome UX Report / CrUX) captures actual user experiences across all real devices, connections, and geographic locations — and is what Google uses for ranking.

Important: Google Search Console's Core Web Vitals report uses field data, not lab data. A page can pass lab tests but fail in the field if real users have slow connections, older devices, or if there are dynamic content or A/B testing variations affecting performance. Use this core web vitals checker for instant heuristic analysis and debugging, then verify real-user data in Google Search Console. At least 75% of page loads must fall in the Good range for all three metrics to pass Google's Core Web Vitals assessment. Also run a mobile-friendly test alongside, since CWV scores are measured separately for mobile and desktop.

Frequently Asked Questions

FAQs on Core Web Vitals Test

Everything you need to know about running a core web vitals test, understanding LCP, INP, CLS, and fixing your CWV score.

What is a core web vitals test and why does it matter for SEO?

A core web vitals test measures Google's three user experience metrics — LCP (Largest Contentful Paint), INP (Interaction to Next Paint), and CLS (Cumulative Layout Shift) — that form part of Google's Page Experience ranking signal. Google confirmed in May 2021 that Core Web Vitals are ranking factors. Pages that pass all three metrics in the Good range have a ranking advantage in both mobile and desktop search. Running a regular CWV check helps you catch and fix performance regressions before they cost you rankings and traffic.

What are the Core Web Vitals thresholds for Good, Needs Improvement, and Poor?

Google's core web vitals thresholds are: LCP — Good: under 2.5s, Needs Improvement: 2.5s–4s, Poor: over 4s. INP (replaced FID in March 2024) — Good: under 200ms, Needs Improvement: 200ms–500ms, Poor: over 500ms. CLS — Good: under 0.1, Needs Improvement: 0.1–0.25, Poor: over 0.25. To pass Google's Core Web Vitals assessment, at least 75% of real user page loads must fall in the Good range for all three metrics based on the Chrome UX Report (CrUX) data.

What is LCP and how do I fix a slow LCP score?

LCP (Largest Contentful Paint) measures when the largest visible element finishes rendering. Poor LCP (over 4s) is the most common CWV failure. The highest-impact LCP fixes are: (1) Preload your hero image with <link rel=preload as=image>. (2) Convert to WebP/AVIF (25–50% smaller). (3) Enable GZIP or Brotli compression on your server. (4) Use a CDN. (5) Eliminate render-blocking CSS/JS. (6) Inline Critical CSS. LCP is the metric where server-side optimisations like compression and caching have the most direct and measurable impact.

What is CLS and what are the most common causes of layout shift?

CLS (Cumulative Layout Shift) measures how much elements unexpectedly shift during load. The most common causes: images without width/height attributes — the single most common CLS failure, fix by adding explicit dimensions to every img tag. Ads without reserved space — set min-height on ad containers. Web fonts causing FOIT — use font-display:swap and preload fonts. Dynamically injected content — avoid inserting elements above existing content after load. A CWV test like this one detects CLS risk factors even without running the page in a real browser.

What is INP and how is it different from FID?

INP (Interaction to Next Paint) replaced FID (First Input Delay) as a Core Web Vitals metric in March 2024. FID only measured the delay before the browser started processing the first interaction. INP measures the full delay from any interaction — not just the first — to the next visual update, throughout the entire session. This makes INP a much stricter responsiveness measure. Good INP = under 200ms. The primary cause of poor INP is long JavaScript tasks blocking the main thread. Fix by adding async/defer to scripts, code-splitting bundles, and reducing event handler complexity.

What is TTFB and why does it matter for Core Web Vitals?

TTFB (Time to First Byte) measures server response time — how long until the first HTML byte arrives after a request. While not itself a Core Web Vitals metric, slow TTFB cascades into poor LCP because the browser can't start parsing HTML or loading resources until bytes arrive. Google's Good TTFB threshold is under 200ms. Fix slow TTFB with server-side caching, CDN edge serving, database query optimisation, and faster hosting. This core web vitals checker measures real TTFB server-side for every URL tested.

How do Core Web Vitals affect Google rankings in 2026?

Core Web Vitals are part of Google's Page Experience signal, a confirmed ranking factor since August 2021. In 2026, CWV data is used in Google Search ranking for both mobile and desktop searches. Google uses real-user field data from the Chrome UX Report (CrUX) for ranking — not lab data from tools. The full Page Experience assessment requires passing CWV plus HTTPS, mobile-friendliness, no intrusive interstitials, and safe browsing. Use Google Search Console's Core Web Vitals report to see your real-user data, grouped by URL pattern.

How do I check core web vitals for free?

You can check core web vitals for free using: (1) This tool — instant heuristic CWV analysis with 20+ checks, no login needed. (2) Google Search Console (Core Web Vitals report) — real-user CrUX field data for your entire site. (3) PageSpeed Insights (pagespeed.web.dev) — both lab (Lighthouse) and field (CrUX) data per URL. (4) Chrome DevTools Performance panel — real-time LCP, CLS, INP measurement in lab conditions. For ranking purposes, Google Search Console field data is the most authoritative — use this checker for instant diagnosis and debugging, then verify in Search Console.

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Behind the Search — Performance SEO & AI Intelligence

Built for the Era of Core Web Vitals, AI Search & Page Experience Rankings

Behind the Search builds SEO tools at the intersection of technical performance, Core Web Vitals, and AI search engine discoverability. As Google's ranking algorithm increasingly weights Page Experience signals — and as AI Overviews, ChatGPT, Claude, and Perplexity crawl the web to generate answers — technical excellence is now both an SEO and an AEO requirement.

Our core web vitals test is designed around one principle: show the exact technical truth, not a vanity score. We check real TTFB, real HTTP headers, real markup patterns — and give you the specific fix for every issue found. No inflated scores, no paywalled results, no account required.

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