Core Web Vitals Score Checker (LCP, INP, CLS)

Understanding the Core Web Vitals checker — concept and principle

Core Web Vitals are Google's three user-experience metrics for web pages: Largest Contentful Paint (loading), Interaction to Next Paint (responsiveness) and Cumulative Layout Shift (visual stability). This checker rates each value you enter against Google's published thresholds and also rates two supporting diagnostics, Time to First Byte and First Contentful Paint. Developers and SEO teams use it to read field data from tools such as PageSpeed Insights or the Chrome UX Report.

Green, amber, red bands for each metric; the default values all sit in the green.

How it is calculated

Each metric is compared with two cut-offs: at or below the first is good, at or below the second is needs improvement, above it is poor.

LCP: good ≤ 2.5 s, poor > 4.0 s.
INP: good ≤ 200 ms, poor > 500 ms.
CLS: good ≤ 0.1, poor > 0.25 (a unitless score).
TTFB: good ≤ 800 ms, poor > 1800 ms.
FCP: good ≤ 1.8 s, poor > 3.0 s.

The overall verdict is "passes" only when LCP, INP and CLS are all good; TTFB and FCP are shown for diagnosis and do not affect it.

The principle

Google sets thresholds where research and real-world data suggest most users perceive the page as fast and stable. The assessment uses the 75th percentile of real page visits, split by mobile and desktop, so a page passes only if at least three in four visits are good. LCP marks when the largest image or text block is rendered. INP observes the latency of clicks, taps and key presses throughout the visit and reports one of the slowest. CLS adds up unexpected layout shifts, each scored as impact fraction times distance fraction, taking the worst burst of shifts. INP replaced First Input Delay as a Core Web Vital in March 2024.

Worked example

Inputs: LCP 2.1 s, INP 180 ms, CLS 0.08, TTFB 600 ms, FCP 1.5 s.
LCP 2.1 ≤ 2.5 → good; INP 180 ≤ 200 → good; CLS 0.08 ≤ 0.1 → good.
TTFB 600 ≤ 800 → good; FCP 1.5 ≤ 1.8 → good.
All three core metrics are good, so the overall result is passes (all good).

How to read the result

Enter p75 field values, not a single lab run. A slow TTFB usually explains a slow FCP and LCP, so fix the server or caching first. A poor INP points to long JavaScript tasks on the main thread; a poor CLS points to images, ads or fonts without reserved space.

Common mistakes

Using one Lighthouse lab score; lab tests cannot measure real INP.
Entering the average instead of the 75th percentile.
Mixing mobile and desktop data, which are assessed separately.

Keep in mind: Passing these thresholds is one signal of page experience, not a guarantee of rankings, and real users on slow devices may still see problems.

Frequently asked questions

Is FID still a Core Web Vital?

No. INP replaced FID in March 2024 because it measures all interactions, not only the first input delay.

Why is CLS not in seconds?

It is a score that multiplies how much of the viewport moved by how far it moved, so it has no unit.

Where do I find my field data?

PageSpeed Insights and Search Console show Chrome UX Report data for pages and origins with enough traffic.

Sources: web.dev, "Web Vitals" and the individual LCP, INP, CLS, TTFB and FCP metric pages (Google) · Chrome UX Report documentation, developer.chrome.com

Checked 8 Nov 2026. Standard formulas and RFC/IEEE definitions, computed locally in your browser — nothing you type is sent anywhere.

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