What Are Core Web Vitals
What Core Web Vitals Measure
Core Web Vitals are a set of three metrics from Google that measure real user experience on a web page: how quickly the main content appears, how quickly the page responds to input and how stable the layout stays while loading. They are a focused subset of the many performance measures available, chosen because they match things visitors notice.
The three metrics are Largest Contentful Paint (LCP), which measures loading, Interaction to Next Paint (INP), which measures responsiveness, and Cumulative Layout Shift (CLS), which measures visual stability. Each has its own topic, so this article focuses on how the set works as a whole.
Good, Needs Improvement and Poor Thresholds
Each metric is rated against three bands. For LCP, a result of 2.5 seconds or less is good and over 4 seconds is poor. For INP, 200 milliseconds or less is good and over 500 milliseconds is poor. For CLS, a score of 0.1 or less is good and above 0.25 is poor. Results between the two thresholds need improvement.
The thresholds reflect how people perceive delays and shifts, and they are the same for every site. Google documents them on its developer website. They work as a shared reference point, even when your own goals are stricter.
The 75th Percentile Rule
Core Web Vitals are not judged on an average or on one visit. They are assessed on field data, meaning measurements from real Chrome users collected over a rolling 28-day period. For each metric, the value at the 75th percentile is compared with the thresholds. In other words, at least three out of four page views must meet the good threshold for the page to pass that metric.
The rule is deliberately demanding. A site cannot pass simply because most visitors have fast devices, since the slowest quarter still counts. Assessment is done separately for mobile and desktop, so a site can pass on one and fail on the other. The difference between this kind of data and lab tests is explained in field data versus lab data.
A News Site Example
Consider a news site whose article pages load a large hero image, several advertising scripts and a comments widget. In a lab test on a fast laptop, the page looks good. Field data from readers on mid-range phones tells a different story: the hero image appears late, the page jumps as ads arrive and taps feel slow. Core Web Vitals reflect that second picture, because it comes from the visitors who actually arrive.
Frequently asked questions
Do Core Web Vitals scores come from my own tests?
Not usually. Official assessments use field data from Chrome users who visited the page. Lab tests such as Lighthouse simulate one device and connection. They are useful for finding likely causes, but they cannot show the experience of your full audience. If a site has too little traffic to produce field data, lab tests are the main measurement available.
Which Core Web Vital matters most?
All three are part of the assessment, and a page must pass each one to meet the standard. Their relative importance depends on the site. A photo-led store may struggle most with LCP, a web application with many interactions with INP, and a site full of late-loading advertisements with CLS. Start by finding which metric fails.