Web Vitals tester online

Web Vitals tester

Analyze your Web Vitals score and diagnostic

Check the performance of your web site: Largest Contentful Paint (LCP), First Input Delay (FID) and Cumulative Layout Shift (CLS)

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What are Web Vitals?

Why should you care?

Web Vitals is a recent Google initiative to provide unified guidelines for measuring essential quality elements to deliver an exceptional user experience on websites.

Google has provided a number of tools over the years to measure and track a site's performance. Some developers have experience in using these tools, while others have found it difficult to keep up with the abundance of tools and new metrics.

Google's Web Vitals initiative aims to simplify the landscape and help web developers focus on the metrics that matter most, the Core Web Vitals.

With Web Vitals, Google is specifying a set of 3 distinct performance metrics to make up their "Core Web Vitals" that they use to make a judgement on whether or not your website has a good user experience. Google is also specifying a few leading indicator metrics in the Web Vitals specification that are not classified as "core."

Google has been promoting the importance of website speed since their inception, so no surprise here. However, this is the first time they explicitly identify the specific performance metrics that are used to drive their search rankings.

Core Web Vitals

What do I need to know about Web Vitals?

The Core Web Vitals focus on three simple UX characteristics of a page:

  1. Loading Times
  2. Interactivity (i.e page responsiveness)
  3. Visual Page Stability

The math behind the metrics can get tricky, but an understanding of what they're trying to accomplish may help simplify things.

The Core Web Vitals metrics are as follows:

  • Largest Contentful Paint (LCP)
  • First Input Delay (FID) / Total Blocking Time (TBT)
  • Cumulative Layout Shift (CLS)

These Core Web Vitals are outcome metrics and a combination of other metrics may be used to act as leading indicators to predict user experience given a change.

Largest Contentful Paint (LCP): measures loading performance. To provide a good user experience, LCP should occur within 2.5 seconds of when the page first starts loading.

First Input Delay (FID): measures interactivity. To provide a good user experience, pages should have a FID of less than 100 milliseconds.

Cumulative Layout Shift (CLS): measures visual stability. To provide a good user experience, pages should maintain a CLS of less than 0.1.

Other Web Vitals

What do I need to know about Web Vitals?

These other Web Vitals often serve as proxy or supplemental metrics for the Core Web Vitals, to help capture a larger part of the experience or to aid in diagnosing a specific issue.

Time To First Byte (TTFB): this audit fails when the browser waits more than 600 ms for the server to respond to the main document request. Users dislike when pages take a long time to load.

First Contentful Paint (FCP): measures the time from when the page starts loading to when any part of the page's content is rendered on the screen.

Time To Interactive (TTI): measures the time from when the page starts loading to when its main sub-resources have loaded and it is capable of reliably responding to user input quickly.

Total Blocking Time (TBT): measures the total amount of time between First Contentful Paint (FCP) and Time to Interactive (TTI) where the main thread was blocked for long enough to prevent input responsiveness. The main thread is considered "blocked" any time there's a task thet runs more than 50 milliseconds.

First Meaningful Paint (FMP): deprecated - measures when the primary content of a page is visible to the user. The raw score for FMP is the time in seconds between the user initiating the page load and the page rendering the primary above-the-fold content.

Speed Index (SI): measures how quickly content is visually displayed during page load. Lighthouse first captures a video of the page loading in the browser and computes the visual progression between frames.

First CPU Idle (FCI): deprecated - measures how long it takes a page to become minimally interactive. A page is considered minimally interactive when most—but not necessarily all—UI elements on the screen are interactive, and the page responds, on average, to most user input in a reasonable amount of time.

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