Core Web Vitals explained
Google's three key speed measurements explained in plain English — what they measure, what the targets are, and why they matter for your site.
Core Web Vitals are three measurements Google uses to judge how good your page feels to a real visitor. They are also ranking signals — a site that passes them can rank higher than one that does not. This guide explains each one clearly.
Quick summary
Google measures three things: how fast your main content loads (LCP), whether your page moves around unexpectedly (CLS), and how quickly it responds to clicks and taps (INP). Each has a target range — green means good, red means needs work.
The three Core Web Vitals
1. Largest Contentful Paint (LCP) — loading speed
LCP measures how long it takes for the largest visible piece of content to appear on screen. This is usually your hero image, a large heading, or a banner.
Think of it as: "When does the visitor see the main thing they came for?"
| Result | Score |
|---|---|
| Under 2.5 seconds | Good |
| 2.5 – 4.0 seconds | Needs improvement |
| Over 4.0 seconds | Poor |
What causes a bad LCP? Usually a large, unoptimized hero image, slow hosting, or a heavy page that has to load many files before it can display anything.
2. Cumulative Layout Shift (CLS) — visual stability
CLS measures how much the page layout jumps around while it is loading. You know the experience: you go to click a button, something loads above it at the last second, and you click the wrong thing.
Think of it as: "Does the page stay still as it loads?"
| Result | Score |
|---|---|
| Under 0.1 | Good |
| 0.1 – 0.25 | Needs improvement |
| Over 0.25 | Poor |
CLS is measured as a score (not seconds). Lower is better. A score of 0 means nothing moved at all.
What causes a bad CLS? Images without defined dimensions (so the browser does not reserve space), ads that load after the page, and web fonts that swap in late.
3. Interaction to Next Paint (INP) — responsiveness
INP measures how quickly the page responds after you click, tap, or press a key. If you tap a button and nothing happens for a second, that is bad INP.
Think of it as: "Does the page feel alive and responsive?"
| Result | Score |
|---|---|
| Under 200 milliseconds | Good |
| 200 – 500 milliseconds | Needs improvement |
| Over 500 milliseconds | Poor |
What causes a bad INP? JavaScript that runs too much work when you interact with the page. Heavy plugins are often the culprit.
INP replaced FID in 2024
You may have heard of "First Input Delay" (FID). Google retired FID in March 2024 and replaced it with INP, which measures all interactions — not just the very first one.
How Google uses Core Web Vitals
Google uses Core Web Vitals as part of its "page experience" ranking signal. A page that passes all three in the green zone gets a small boost compared to a similar page that fails them.
This is not a magic ranking formula. Great content and strong SEO still matter more. But Core Web Vitals are a real factor, and failing them is a disadvantage.
How to check your Core Web Vitals
The easiest way is Google PageSpeed Insights (free):
Go to PageSpeed Insights. Visit pagespeed.web.dev in your browser.
Enter your URL. Type or paste your website address and press Analyze.
Look at the Core Web Vitals section. Each metric shows green (good), orange (needs improvement), or red (poor). Field data (from real visitors) is shown above lab data (simulated test).
See How to measure your site's speed for more tools and a deeper walkthrough.
Common questions
Related guides
- Why website speed matters
- How to measure your site's speed
- Images & page speed
- How hosting affects speed
- Site speed & SEO
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