What are Core Web Vitals?
Definition Core Web Vitals are three metrics Google uses to measure real-world user experience: Largest Contentful Paint (LCP) for loading, Interaction to Next Paint (INP) for responsiveness and Cumulative Layout Shift (CLS) for visual stability. A page passes when, at the 75th percentile of visits, LCP is at most 2.5 s, INP at most 200 ms and CLS at most 0.1. INP replaced First Input Delay (FID) in March 2024.
How to fix it: Largest Contentful Paint: How to Improve LCP on Mobile
The three metrics
| Metric | Measures | Good | Poor |
|---|---|---|---|
| LCP | When the largest image or text block appears | ≤ 2.5 s | > 4 s |
| INP | How fast the page responds to taps, clicks and keys | ≤ 200 ms | > 500 ms |
| CLS | How much visible content jumps around | ≤ 0.1 | > 0.25 |
Values in between are "needs improvement". The set is reviewed over time: Google announced INP as the replacement for First Input Delay in 2023, and the switch took effect on March 12, 2024.
Field data vs lab data
Core Web Vitals are defined on field data: measurements from real visits, assessed at the 75th percentile and separately for mobile and desktop. Google's public field data, the Chrome UX Report (CrUX), comes only from Chrome users who opted in, and it is what PageSpeed Insights and Search Console show. A page needs enough traffic to appear in it.
Lab data comes from loading the page once under controlled conditions, as Lighthouse does. It's repeatable and good for debugging, but it isn't the score Google assesses. Lab tools can measure LCP and CLS; INP needs real interactions, so lab tools use proxies such as Total Blocking Time.
Do Core Web Vitals affect rankings?
Google says Core Web Vitals are used by its ranking systems as part of page experience, but that great page experience doesn't override relevant content. Treat them as a user experience target: slow, jumpy, unresponsive pages lose visitors whatever their ranking.
On iPhone vs Android
The Layout Instability API behind CLS is available only in Chromium browsers, and CrUX data comes only from Chrome. So your Core Web Vitals reflect Android and desktop Chrome visitors, not Safari users on iPhone. Check iPhone performance separately on a real device.
How MobileCheck checks it
MobileCheck reports lab measurements of LCP and CLS for each device and flags LCP over 2.5 s (poor over 4 s) and CLS over 0.1 (poor over 0.25), along with page weight over 3 MB. It does not measure INP, because that needs real visitor interactions.
Related terms
Guides and tools
- Largest Contentful Paint: How to Improve LCP on Mobile
- Cumulative Layout Shift: How to Fix CLS on Mobile
- Page Weight: How to Reduce Page Size for Mobile