Core Web Vitals
Google publishes three field metrics for loading experience. They are not a design score. They are thresholds in Google’s own documentation, and they can be measured on a live site.
Three numbers, not one score
LCP measures when the main content appears. INP measures the delay between a tap and the next paint. CLS measures how much the layout shifts while loading. Google replaced FID with INP as the responsiveness metric.
Google’s published “good” thresholds are LCP within 2.5 seconds, INP within 200 milliseconds, and CLS within 0.1. Above 4 seconds LCP, 500 milliseconds INP, or 0.25 CLS, Google classifies the experience as poor.
| Metric | What it measures | Good | Poor |
|---|---|---|---|
| LCP | When the main content appears | Within 2.5s | Over 4s |
| INP | Response to a tap | Within 200ms | Over 500ms |
| CLS | Layout shift | Within 0.1 | Over 0.25 |
What to do with the number
Measure the live URL on a phone, not a promise in a deck. An image without dimensions shifts the page and raises CLS. A heavy script before the content stretches LCP. A loader that hides the heading does not improve the metric. It delays it.