OneTaskBy Tim King · Updated
Core Web Vitals Calculator
Check LCP, INP and CLS Against Google's Thresholds, and What to Fix First
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How the Core Web Vitals Calculator works
This Core Web Vitals calculator rates LCP, INP and CLS against the thresholds. Core Web Vitals are the three user-experience metrics Google folds into search ranking: how fast the main content appears, how quickly the page responds to a tap or click, and how much the layout jumps around. Enter the values from Search Console, PageSpeed Insights or the Chrome User Experience Report to see each one rated, how far it is from the line, and whether the page as a whole passes.
Core Web Vitals calculator: the three thresholds
Largest Contentful Paint is the time until the largest visible element — usually the hero image, a video poster or the main heading — has rendered. Good is 2.5 seconds or less; poor is over 4 seconds. It is mostly about the server response time, render-blocking CSS and how the largest image is delivered.
Interaction to Next Paint replaced First Input Delay in 2024. It measures the worst-case delay between a user interaction and the next frame being painted, across the whole visit, not just the first tap. Good is 200 ms or less; poor is over 500 ms. It is almost entirely about JavaScript on the main thread: long tasks, heavy event handlers, hydration and third-party scripts.
Cumulative Layout Shift is a score for how much visible content moved without the user causing it — images loading without reserved space, ads and banners pushing content down, fonts swapping in at a different size. Good is 0.1 or less; poor is over 0.25. It has no unit, since it is the fraction of the viewport affected multiplied by the distance moved.
The 75th percentile. All three are assessed on real users' page loads from Chrome, at the 75th percentile — the value that three quarters of visits beat. A median that looks fine can still fail if the slowest quarter of visits, typically on older phones and weaker connections, are poor. The page passes only when all three core metrics are good at p75; a single poor metric fails it, and there is no partial credit. The numbers Search Console shows are these p75 values, grouped by URL pattern rather than individual page.
FCP and TTFB are not Core Web Vitals but are reported alongside them because they explain the others. A slow Time to First Byte delays everything downstream; if TTFB is 1.5 seconds, an LCP under 2.5 is nearly impossible. First Contentful Paint bounds how early LCP can be. Both have their own good and poor thresholds and are rated here for that reason, but they do not affect the pass or fail.
Lab versus field. Lighthouse gives lab values from a simulated device; Search Console and the CrUX dashboard give field values from real users. They differ, sometimes a lot, and the field data is what ranking uses. INP in particular cannot be measured in a lab run without interactions, which is why Lighthouse reports Total Blocking Time instead. The Lighthouse Score Explainer on this site covers the lab score.
FAQ
My LCP is 2,600 ms. Is that a fail?
It is "needs improvement", which fails the assessment. The good threshold is a hard line at 2,500 ms at the 75th percentile. Being 100 ms over is a much easier fix than being 2 seconds over, but the page does not pass until it is under.
Why does Search Console say failing when PageSpeed says the metrics are fine?
Search Console uses field data grouped by similar URLs over the previous 28 days; PageSpeed Insights shows both a lab run and, when there is enough traffic, field data for that URL or its origin. A recent fix takes up to 28 days to clear from the field numbers, and a group can be dragged down by other pages in it.
What happened to First Input Delay?
It was replaced by INP in March 2024. FID only measured the delay before the first interaction started processing; INP measures the full time to the next paint for every interaction and takes the worst, so it is much harder to pass and much more representative of how a page feels.
Do Core Web Vitals matter for ranking?
They are a ranking signal, a modest one, and a tiebreaker rather than a decider; content relevance dominates. They matter more for the experience itself — slow, jumpy pages lose visitors before ranking is even a question.
How do I get the values for my own site?
Search Console's Core Web Vitals report, PageSpeed Insights (the "Discover what your real users
are experiencing" section), or the web-vitals JavaScript library sending to your own analytics.
Lighthouse gives lab values for LCP and CLS but not INP.
See also: all Dev Utilities tools, Formatters & Validators, Data & Encoding, Security & Identifiers, SEO, Accessibility, AI.
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