Guide · Search and speed
Core Web Vitals: What LCP, FID and CLS Mean for Your Site
Core web vitals are three Google measures of how a page loads, responds and holds still. This guide explains each one in plain words, gives the target to aim for, and shows how to check your own site for free.
What the three measures are
Core Web Vitals are three timed checks that show how a page feels to a real visitor. Google picked them to cover loading, response and stability. Google says they apply to all web pages and that site owners should measure them.
The three are Largest Contentful Paint (LCP), First Input Delay (FID) and Cumulative Layout Shift (CLS). Google also says the set will change over time. The current set is the one for 2020.
Each one comes with a target. For scoring, Google points to the 75th percentile of real page loads, and it asks for phone visits and computer visits to be tallied apart. Put simply, at least three in every four visits should hit the target. A page passes only when it meets all three.
Sources: web.dev, Web Vitals
LCP: how fast the main part loads
LCP measures loading speed. It marks the moment the biggest piece of content in view, such as a large photo or a block of text, has appeared.
The target is 2.5 seconds or less from when the page first starts loading. A slow server, a heavy hero image or code that blocks the page can all push LCP past that.
Think of a shop door that opens, but the shelves show up three seconds later. Visitors notice the wait, even if the page works fine after that.
Sources: web.dev, Web Vitals
FID: how fast the page reacts
FID measures response. It is the time between a visitor's first tap or click and the moment the browser can start to act on it.
The target is under 100 milliseconds. That is one tenth of a second. A page that is busy running scripts can ignore a tap for longer than that, which feels like a frozen button.
One catch is that FID needs a real person to tap. Lab tools such as Lighthouse load a page with no visitor, so they cannot measure FID. Google points to a lab measure called Total Blocking Time (TBT) as a close stand-in. Work that cuts TBT in the lab should also help FID for real users.
CLS: how steady the page stays
CLS measures visual stability. It adds up how much page content jumps around while the page loads.
The target is a score under 0.1. You have seen a bad score if you ever went to tap a link and an ad loaded above it, pushing the link down so you hit the wrong thing.
Common causes are images with no set size, ads or embeds that appear late, and fonts that swap in and change the text size. Setting a width and height for images and ad slots is a simple first fix.
Sources: web.dev, Cumulative Layout Shift; web.dev, Web Vitals
How to check your own numbers for free
You do not need to pay for a tool. Google lists these free options, split into two groups.
- Field tools use data from real visits: the Chrome User Experience Report, PageSpeed Insights and the Core Web Vitals report in Search Console.
- Lab tools test a page in a controlled setup: Chrome DevTools and Lighthouse.
Start with the Core Web Vitals report in Search Console. It groups your pages and shows which ones need work. Then run a slow page through PageSpeed Insights to see what to fix first.
Field data shows what visitors really got. Lab data helps you test a fix before you ship it. Use both.
Sources: web.dev, Web Vitals
Why it matters for search in 2021
In November 2020, Google said the page experience signals would roll out in ranking in May 2021. These signals combine Core Web Vitals with mobile-friendliness, safe browsing, HTTPS and rules on intrusive pop-ups.
Google also said that from May 2021, pages with great page experience can be prioritised for the mobile Top Stories feature, whether they use AMP or another technology.
That gives site owners a few months. A sensible plan is to check the Search Console report this month, list the slowest templates, and fix the biggest problem first. Good content still matters, and a fast page does not replace it.
Sources: Google Search Central blog, Timing for bringing page experience to Google Search
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