core web vitals

Core Web Vitals are three measurable numbers Google uses to judge the real-world user experience of a web page: Largest Contentful Paint (LCP), how quickly the main content appears; Interaction to Next Paint (INP), how responsive the page feels when clicked or tapped; and Cumulative Layout Shift (CLS), how much the layout jumps around while loading. A page passes when 75% of real visits, measured over 28 rolling days, hit the 'good' threshold on all three. The thresholds are LCP under 2.5 seconds, INP under 200 milliseconds, and CLS below 0.1. Google made Core Web Vitals a ranking signal in 2021 and upgraded INP to replace the earlier First Input Delay metric in March 2024. The measurement comes from Chrome users' actual sessions — not synthetic tests — which is why a site can score 100 in Lighthouse and still fail CrUX field data.

What a passing site actually looks like

This site passes all three on a mid-range Android over a throttled 4G connection — LCP around 1.1 seconds, INP under 100 milliseconds, CLS close to zero. The gains come from static generation, self-hosted fonts with font-display: swap, explicit width and height on every image, and almost no client-side JavaScript. A typical Squarespace template on the same device ships CLS above 0.25 and LCP above 3 seconds, mostly because of late-loading hero images and injected third-party scripts.

Why a passing score earns its keep

Core Web Vitals are the closest thing Google has to a public SEO scorecard. A failing site pays for that failure in two ways — slightly lower search rankings, and measurably higher bounce rates from people who give up before the page renders. For a small business, the difference between passing and failing is often the difference between a £50 Google Ads click converting and the same click bouncing before the hero image even arrives.

Related terms

• lighthouse scoreChrome's free 0-to-100 audit across Performance, Accessibility, Best Practices, and SEO. Shorthand for ‘is this well-built’.
• LCP (Largest Contentful Paint)Largest Contentful Paint — how long the biggest visible element takes to render. Google's threshold is 2.5 seconds.
• INP (Interaction to Next Paint)Interaction to Next Paint — how responsive the page feels when you click or tap. Google's threshold is 200 milliseconds.
• CLS (Cumulative Layout Shift)Cumulative Layout Shift — how much the page jumps around while loading. Google's threshold is a score of 0.1.