Core Web Vitals in 2026: What They Are and How to Test Them for Free
LCP, INP, CLS - three acronyms that have helped decide how well your website ranks on Google since 2021. Here's what they measure, what counts as "good," and how to test them for free in under 2 minutes.
What are Core Web Vitals?
Core Web Vitals are three metrics Google uses as an official part of how it evaluates search results to measure how a website feels to real users - not just how fast it technically loads, but how fast it feels usable. They've fed directly into rankings since 2021, as part of what Google calls "page experience."
The key difference from older loading-speed metrics: Core Web Vitals aren't just measured in the lab (Lighthouse simulation) - they're also collected as real field data from the Chrome browsers of actual visitors. A page can score well in a lab test and still perform poorly for real users if those users are on slower devices or connections. We break down, with numbers, how much slow load times actually hurt rankings in our article on the 10 most common SEO mistakes.
The 3 Core Web Vitals in detail
All three values need to be "good" at the 75th percentile for Google to rate the page as "good" overall.
Largest Contentful Paint
Loading speed - how fast the largest visible content (usually an image or text block) loads.
Interaction to Next Paint
Responsiveness - how quickly the page visually reacts to clicks, taps, or keyboard input.
Cumulative Layout Shift
Visual stability - how much elements unexpectedly shift while the page is loading.
Worth knowing if you're reading older articles
INP replaced FID (First Input Delay) as the official Core Web Vital in March 2024. Many older SEO guides still mention FID - that's outdated. Only check INP today.
How do you test your Core Web Vitals for free?
The fastest way: PageSpeed Insights from Google itself. Enter a URL, wait, done - you get both lab data (Lighthouse simulation) and real field data from the Chrome User Experience Report, provided the page gets enough traffic.
Alternatives for manual checking: Chrome DevTools (the Lighthouse tab, right in the browser) for lab data, or the Core Web Vitals report in Google Search Console for aggregated field data over the last 28-day period.
The catch with all three: they each check only one URL per run. If you want to see multiple subpages at once - product pages, blog posts, landing pages - you either need a lot of manual time or an automated tool that crawls multiple pages.
The most common causes of poor scores
Most common causes of poor LCP
- Uncompressed or poorly formatted images (no WebP/AVIF)
- Render-blocking CSS and JavaScript in the <head>
- Slow server response time (TTFB over 800ms)
- Missing preload for the most important above-the-fold image
Most common causes of poor INP
- Large, blocking JavaScript bundles that occupy the main thread
- Heavy third-party scripts (tracking, chat widgets, ads)
- Too many DOM elements that need to be recalculated on interaction
- Missing code-splitting - the whole page loads JS that only part of it needs
Most common causes of poor CLS
- Images and videos without fixed width/height attributes
- Web fonts without font-display: optional or swap (font swaps shift text)
- Dynamically loaded content (banners, cookie notices) without reserved space
- Ad containers without a defined minimum height
Frequently asked questions about Core Web Vitals
What are Core Web Vitals?
Core Web Vitals are three metrics defined by Google that measure a website's perceived user experience: LCP (loading speed), INP (responsiveness), and CLS (visual stability). They've been an official Google ranking factor since 2021.
How do I test my Core Web Vitals for free?
The fastest way is Google's own PageSpeed Insights (pagespeed.web.dev) - enter a URL, get results in seconds. For checking multiple pages at once, an automated SEO test tool like AuditAI works better, since PageSpeed Insights only checks one URL per run.
What is the difference between INP and the old FID?
FID (First Input Delay) only measured the delay until the first reaction to a user interaction. INP (Interaction to Next Paint) replaced FID as the official Core Web Vitals metric in March 2024 and measures responsiveness across a visitor's entire time on the page - a much more complete picture of actual interactivity.
How often should I check Core Web Vitals?
After every major deployment, and at least once a month, since new images, scripts, or third-party embeds can quietly degrade your scores. If you need to keep an eye on Core Web Vitals across multiple pages, an automated, recurring check beats manual one-off testing.
Check Core Web Vitals across every subpage
AuditAI measures LCP, TTFB, FCP, and load time across up to 25 subpages at once - alongside SEO and GEO signals in the same report. Start without registration, sign up free for the full report with all scores.
Test performance nowThe SEO Checklist 2026 in 15 Minutes
Core Web Vitals is just one of 6 phases - the complete self-check sequence, including GEO signals.