Core Web Vitals: definition, metrics and SEO impact
Updated on February 22, 2026
Quick definition
Core Web Vitals are a set of three metrics defined by Google to objectively measure real user experience on a web page: LCP (loading speed), INP (responsiveness to interactions), and CLS (visual stability). Core Web Vitals are official ranking factors integrated into Google's algorithm since 2021.
How it works
Core Web Vitals were introduced by Google in 2020 and became an official ranking factor in May 2021. These three metrics measure concrete aspects of user experience:
- LCP (Largest Contentful Paint) — main-content display speed: good if under 2.5 s, needs improvement between 2.5 s and 4 s, poor beyond 4 s
- CLS (Cumulative Layout Shift) — visual stability: good if under 0.1, needs improvement between 0.1 and 0.25, poor beyond 0.25
- INP (Interaction to Next Paint) — responsiveness to interactions (replaced FID in March 2024): good if under 200 ms, needs improvement between 200 ms and 500 ms, poor beyond
Google measures these metrics on real user data via the Chrome User Experience Report (CrUX), not lab data, and separately for mobile and desktop.
Why it matters
Core Web Vitals represent a page experience signal that Google uses to differentiate between content of equivalent quality. A site that hits all green Core Web Vitals gains a competitive edge on competitive queries.
Beyond SEO, improving Core Web Vitals has a direct impact on conversions: a Google study shows that a site with a good LCP converts 24% better than a site with a poor LCP. These metrics therefore align SEO and UX goals.
How to improve or use it
- 1Measure your Core Web Vitals on real-user data via Google Search Console (Page Experience report) and PageSpeed Insights.
- 2For LCP: compress your images, use modern formats (WebP, AVIF), preload critical resources.
- 3For CLS: set explicit dimensions on images and videos, avoid injecting content above the fold.
- 4For INP: reduce blocking JavaScript, defer non-critical third-party scripts, and optimize event handlers.
With Sublim
Sublim integrates page load performance measurement directly into your analytics dashboard. You see your speed metrics evolve alongside your traffic and conversions, allowing you to directly correlate Core Web Vitals improvements with business impact — all without cookies, in full GDPR compliance.
Frequently asked questions
Are Core Web Vitals the only Google ranking factors?
No, Core Web Vitals are one of many signals used by Google, alongside content relevance, backlinks, HTTPS security, mobile-friendliness and many others. They have more impact when separating pages with equivalent content quality than they do as a substitute for low-quality content.
What is the difference between lab data and field data for Core Web Vitals?
Lab data (such as Lighthouse) simulate page loading under controlled conditions and are useful for debugging. Field data (CrUX — Chrome User Experience Report) reflect real user experiences over the past 28 days. Google uses only field data for ranking.
Do mobile and desktop sites have separate Core Web Vitals?
Yes, Google measures Core Web Vitals separately for mobile and desktop. Ranking evaluation is primarily based on mobile data, since Google uses mobile-first indexing. It is therefore essential to prioritise smartphone experience optimisation.
Related terms
LCP (Largest Contentful Paint) is a [Core Web Vitals](core-web-vitals)…
CLS (Cumulative Layout Shift) is a [Core Web Vitals](core-web-vitals) …
INP (Interaction to Next Paint) is the [Core Web Vitals](core-web-vita…
SEO (Search Engine Optimization) is the set of practices aimed at impr…