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Core Web Vitals checker

The three numbers Google uses to describe page experience — loading, responsiveness and visual stability — measured on your page and explained in plain English.

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What we check and why

The three Core Web Vitals

Google assesses each metric at the 75th percentile of real visits, so three out of four visitors need a good experience for the page to pass.

Largest Contentful Paint (LCP) measures loading: when the largest image or text block in the first screen is rendered. Good is 2.5 seconds or less.
Interaction to Next Paint (INP) measures responsiveness: how quickly the page reacts visibly after a tap, click or key press. Good is 200 milliseconds or less. It replaced First Input Delay in March 2024.
Cumulative Layout Shift (CLS) measures visual stability: how much content jumps while loading. Good is 0.1 or less.

Lab test versus field data

This checker runs Google Lighthouse in headless Chrome on our servers, once with a throttled mobile profile and once on desktop. That is a lab test: the same conditions every time, which makes it ideal for finding causes and checking fixes. INP needs real user interactions, so the lab uses Total Blocking Time — how long scripts keep the main thread busy — as its closest stand-in.

Field data comes from real Chrome users and is published in the Chrome UX Report when a site has enough traffic. Many small firm websites do not, which is why a reliable lab test is often the only way to see Core Web Vitals at all. If you do have field data in Google Search Console, use it to confirm that your lab improvements reach real visitors.

What usually goes wrong, and the fix

Slow LCP on firm websites is mostly caused by a large hero photo, a slow server response or render-blocking CSS and fonts. Serve the hero image in WebP or AVIF at the size it is displayed, do not lazy-load it, preload it if needed and cache pages on the server or a CDN.

High blocking time comes from JavaScript: page builders, sliders, chat widgets and tag managers loading many tags. Remove what you do not use and defer the rest. Layout shift comes from images without width and height, late-loading cookie banners and web fonts that swap in with different sizes; reserve space for them. The report lists Lighthouse's opportunities ordered by estimated savings.

Frequently asked questions

Do Core Web Vitals affect Google rankings?

They are part of Google's page experience signals, which Google describes as one factor among many. Relevance and content quality matter more, but a slow page also loses visitors directly.

Why don't you show INP?

INP can only be measured with real interactions. In a lab test Total Blocking Time is the recommended proxy, and reducing it usually improves INP.

Why is mobile so much slower than desktop?

Lighthouse's mobile profile simulates a mid-range phone with a slower processor and a throttled network, which reflects many real visits.

My field data says good but your test says poor. Who is right?

Both can be right. Field data reflects your actual visitors and devices; the lab test applies demanding fixed conditions. Use the lab results to find improvements.