Core Web Vitals
Core Web Vitals are a standardized set of real-world user experience metrics defined by Google that measure web page performance. They evaluate three key aspects of page quality: visual loading speed (Largest Contentful Paint), interactive responsiveness (Interaction to Next Paint), and visual layout stability during rendering (Cumulative Layout Shift).
In plain English
For many years, website performance was measured using technical lab benchmarks, such as total page weight in megabytes or the exact second when the browser fired its window.onload event. However, those numbers frequently failed to reflect the actual experience of human users. A webpage could take six seconds to finish downloading background analytics scripts while displaying its primary reading text almost instantly. Conversely, a page might load in two seconds but freeze completely when a visitor tries to tap a button.
Core Web Vitals represent Google’s initiative to quantify real-world user experience through field data. Sourced directly from millions of real browser sessions via the Chrome User Experience Report (CrUX), these metrics focus on three essential aspects of visiting a webpage: Did the main content load quickly? Did the page respond immediately when I interacted with it? Did elements jump around on the screen while I was reading?
An example
Consider an e-commerce website where user experience directly impacts sales and search rankings:
- Largest Contentful Paint (LCP): This measures visual loading speed. On a poorly optimized product page, a massive, uncompressed product image takes 4.5 seconds to render over a mobile connection, leaving visitors staring at blank space. On an optimized page, the image is compressed with modern formats like WebP, preloaded in HTML headers, and renders in 1.4 seconds, well inside Google’s 2.5-second “Good” benchmark.
- Interaction to Next Paint (INP): This measures interactive responsiveness. When a customer taps an “Add to Cart” button on the slow site, complex JavaScript tasks monopolize the browser’s main thread, causing a noticeable 450-millisecond freeze before the cart updates. On the fast site, work is broken into asynchronous microtasks, allowing the browser to repaint visual feedback within 90 milliseconds.
- Cumulative Layout Shift (CLS): This measures visual stability. On an unoptimized site, images lack explicit aspect ratio dimensions in CSS, and dynamic ad banners load late, abruptly shoving the article text downward mid-sentence and causing the visitor to tap the wrong link. The optimized site reserves layout space in advance, maintaining a stable layout with a CLS score of 0.01.
Google evaluates these metrics at the 75th percentile of all user visits over a rolling 28-day window to determine whether a domain meets quality thresholds.
Why it matters
Core Web Vitals are an official ranking factor within Google’s search algorithms. While authoritative content and topical relevance remain the primary criteria for high search rankings, Core Web Vitals function as a powerful tiebreaker when competing pages offer similar information depth. Websites that pass all three Core Web Vitals metrics deliver lower bounce rates, higher visitor retention, and improved organic search visibility.
Related terms
Read the full guide to technical SEO, or explore our complete guide to Core Web Vitals.