Core Web Vitals Explained: What They Are and How to Check
Core Web Vitals are three measurements of how a page feels to a visitor: how quickly the main content appears, how fast the page reacts when someone taps or clicks, and how much the layout jumps while loading. Google publishes a target for each one, and you can check all three for free in a few minutes.
The three measurements in plain words
- Loading (Largest Contentful Paint, or LCP): how long it takes for the biggest piece of content, often a main photo or a headline block, to appear. The target is 2.5 seconds or less.
- Interaction (Interaction to Next Paint, or INP): how quickly the page responds after a tap, a click or a key press. The target is 200 milliseconds or less.
- Visual stability (Cumulative Layout Shift, or CLS): how much the content moves around while the page loads, for example when a banner pushes the text down just as you try to tap a button. The target is 0.1 or less.
A page counts as passing when it meets the targets for at least three out of four real visits (the 75th percentile). Mobile and desktop are measured separately, so a page can pass on one and fail on the other.
Why they matter for a small site
These measurements describe what a visitor actually experiences. A slow main image, a button that does not react, or a page that shifts under your finger all push people away before they read a word. They are also part of how Google looks at page experience. Good content still matters more than a perfect score, but a site that is pleasant to use gives that content a fair chance. For the wider picture of how search works for a new site, see how Google Search works.
Field data and lab data
Two kinds of data are used, and they answer different questions.
Field data comes from real visitors using real phones and computers. It shows how the site performs for the people who use it.
Lab data comes from a test run on one device under set conditions. It is useful while you are building or fixing a page, because the result appears immediately and you can repeat it. The lab tools cannot measure INP directly, so they use a related measurement as a stand-in, and lab results do not replace field data.
If the two disagree, trust the field data for how visitors really experience the page, and use the lab test to find what to change.
Free ways to check your pages
- PageSpeed Insights. Paste a page address and it shows field data when enough real visits exist, plus a lab test with suggestions.
- The Core Web Vitals report in Search Console. It groups the pages of your site by status for mobile and desktop. Setup is covered in the main Search Console reports.
- Browser developer tools. The built-in audit tool in Chrome runs a lab test on the page you have open.
A new or small site may have too few visits to show field data. In that case rely on the lab test and check again after the site has been live for a while.
The usual causes of poor scores
Most problems on small sites come from a short list. Each one hurts a specific measurement.
| Cause | Measurement it hurts | Typical fix |
|---|---|---|
| Very large images | Loading | Resize to the displayed size and use a modern image format |
| Heavy or many scripts | Interaction | Remove tools you do not use and load the rest later |
| Fonts that load late | Loading and stability | Use fewer fonts and weights |
| Images and banners without reserved space | Stability | Set width and height so the space is held in advance |
| Pop-ups and notices that push content down | Stability | Place them where they do not move the text |
A simple order for fixing a page
- Run the test and write down the three results, for mobile first.
- Start with images, because they are the most common cause of slow loading.
- Remove or delay scripts that add little, such as extra widgets.
- Reserve space for images, ads and banners.
- Test again after each change, so you know what helped.
Field data updates over several weeks, so do not expect the report to turn green the next day. A more detailed walk through speed problems is in responsive design basics, and image handling is covered in the guide to image alt text.
When the measurements change
The set of measurements is not fixed forever. A metric called First Input Delay was retired and replaced by Interaction to Next Paint. Changes are announced in advance, so the targets you work toward stay stable for a good while. Check the official web.dev page on Web Vitals from time to time for updates.
FAQ
Do Core Web Vitals affect rankings?
They are one part of page experience, which search engines consider, but they do not replace useful content. A fast page with weak content will not outrank a helpful one.
What is a good score?
The published targets are 2.5 seconds or less for loading, 200 milliseconds or less for interaction and 0.1 or less for layout shift, measured on at least three out of four visits.
Why do my lab and field results differ?
The lab test runs once on one device, while field data collects real visits on many devices and connections. Use the lab test to find causes and the field data to confirm the result.
Do I need a developer to improve them?
Not always. Resizing images, removing unused tools and reserving space for banners can be done in most website builders. Script and font problems may need a developer.
Why does Search Console show no data for my site?
Field data needs enough real visits. A new site may have none yet, so use the lab test for now and look again later.
Your next step: open PageSpeed Insights, test your homepage on mobile and write down the three results. Fix the biggest image first, then test again.