Core Web Vitals

Updated September 30, 20262 min read

In short

Core Web Vitals are three metrics that measure a page's real-user experience: loading speed (LCP), responsiveness (INP), and visual stability (CLS).

What are Core Web Vitals?

Core Web Vitals are a set of three metrics, defined by Google as part of its Web Vitals initiative, that measure how a page feels to real users. Largest Contentful Paint (LCP) measures loading, meaning how long the biggest image or block of text in the visible area takes to appear. Interaction to Next Paint (INP) measures responsiveness, meaning how quickly the page visibly reacts after a click, tap, or key press, and Cumulative Layout Shift (CLS) measures visual stability, meaning how much content unexpectedly jumps around.

A page passes when at least 75 percent of visits meet the good thresholds: an LCP of 2.5 seconds or less, an INP of 200 milliseconds or less, and a CLS of 0.1 or less. The scores come from field data, meaning measurements collected from real visitors' browsers, and Google Search uses them as one of its page experience ranking signals. INP replaced the older First Input Delay (FID) metric in March 2024.

Think of a restaurant visit: LCP is how soon your food arrives, INP is how quickly the waiter responds when you wave, and CLS is whether someone keeps moving your plate while you eat. Teams improve LCP by optimizing images and server response times, INP by breaking up long JavaScript tasks that block the main thread, and CLS by reserving space for images, ads, and embeds with explicit sizes.

Core Web Vitals are often confused with lab scores from tools such as Lighthouse. Lab tests load a page once on a simulated device and are great for debugging, while Core Web Vitals are based on field data from real visitors over the previous 28 days, so the two can disagree. Lab tools also can't measure INP directly, because it needs real user interactions, so they report related metrics such as Total Blocking Time instead.

Key takeaways

The three Core Web Vitals are LCP (loading), INP (responsiveness), and CLS (visual stability).
Good scores are LCP of 2.5 s or less, INP of 200 ms or less, and CLS of 0.1 or less.
A page passes when at least 75 percent of real visits meet those thresholds.
They are a Google Search ranking signal, but content relevance matters more.
INP replaced First Input Delay (FID) in March 2024.

Example

Measuring Core Web Vitals from real usersjavascript

// Using the open-source web-vitals library
import { onLCP, onINP, onCLS } from "web-vitals";

function sendToAnalytics({ name, value, rating }) {
  // e.g. name: "LCP", value: 1830 (milliseconds), rating: "good"
  navigator.sendBeacon("/analytics", JSON.stringify({ name, value, rating }));
}

onLCP(sendToAnalytics);
onINP(sendToAnalytics);
onCLS(sendToAnalytics);

Readers ask

What are the three Core Web Vitals?

They are Largest Contentful Paint (LCP) for loading speed, Interaction to Next Paint (INP) for responsiveness, and Cumulative Layout Shift (CLS) for visual stability.

Do Core Web Vitals affect SEO?

Yes, they are part of the page experience signals Google uses in search ranking. Their effect is usually smaller than the relevance and quality of the content, so they matter most when competing pages are otherwise similar.

What replaced First Input Delay?

Interaction to Next Paint (INP) replaced First Input Delay (FID) as a Core Web Vital in March 2024. FID measured only the delay before the first interaction was handled, while INP looks at how long interactions take to update the screen throughout the whole visit.