Core Web Vitals checker.
Paste your LCP, INP and CLS from the field data section of PageSpeed Insights. This grades them and names the fixes. It does not crawl your site.
This tool does not crawl or measure your site. It cannot: field data comes from real visitors on real devices, and no page can collect that for a domain it does not run on. Get your numbers from PageSpeed Insights, the Chrome User Experience Report, Search Console, or your own real-user monitoring, then grade them here. Nothing you type is sent anywhere.
What are the Core Web Vitals thresholds?
A page passes when, at the 75th percentile of real visits, Largest Contentful Paint is 2.5 seconds or less, Interaction to Next Paint is 200 milliseconds or less, and Cumulative Layout Shift is 0.1 or less. Anything up to 4 seconds, 500 milliseconds and 0.25 is needs improvement. Above that is poor.
The three metrics, plainly.
Each one measures a different kind of bad experience: waiting, tapping and nothing happening, and the page moving under your thumb.
| Metric | What it measures | Good | Needs improvement | Poor |
|---|---|---|---|---|
| LCP | Time until the largest visible element has rendered | 2.5s or less | 2.5s to 4.0s | Over 4.0s |
| INP | How long the page takes to visibly respond to an interaction | 200ms or less | 200ms to 500ms | Over 500ms |
| CLS | How much visible content moves unexpectedly during load | 0.1 or less | 0.1 to 0.25 | Over 0.25 |
INP replaced First Input Delay as a Core Web Vital in March 2024. FID only measured the delay before the browser started handling the first interaction, which nearly every site passed; INP measures the full round trip to the next paint, for every interaction, which is what a visitor actually feels.
Getting numbers worth grading.
- 01
Open PageSpeed Insights
Enter the URL of a page that gets real traffic. Not your staging site, and not a page nobody visits, because field data needs visitors to exist.
- 02
Read the field data, not the lab score
The top section, labelled as real-user data over the last 28 days, is what counts. The Lighthouse score below it is a simulation and does not include INP.
- 03
Check mobile and desktop separately
They are assessed separately and usually differ a lot. If only one is failing, that is where the work is.
- 04
Grade and fix the worst one
Paste the numbers above. Fix one metric at a time: the causes barely overlap, and changes aimed at LCP rarely help INP.
One reading is a snapshot.
PageSpeed Insights tells you where a page stood over the last 28 days. It does not tell you that INP got worse the day you added a chat widget, or that LCP is fine everywhere except one template.
Mrkr collects Core Web Vitals from your real visitors as part of ordinary analytics, so the numbers sit next to the traffic that experienced them. That is the version you can act on: this page, on mobile, from this country, got slower on Tuesday.
Things that quietly wreck your vitals.
Lazy-loading the hero image
loading="lazy" on the largest above-the-fold image delays the exact element LCP measures.
The cookie banner
Early JavaScript that blocks the main thread and then inserts a large element. It hurts all three at once.
Third-party tags
Chat widgets, heatmaps and tag managers compete for the thread your interactions need.
Fonts that swap late
A font arriving after first paint reflows every line. Preload it and match the fallback metrics.
Slots with no reserved height
A zero-height ad or embed that expands shifts everything below it. Reserve the space in CSS.
Measuring only your own machine
Your laptop is not the 75th percentile. The failures are mid-range Android on mobile networks.
Vitals next to the traffic that felt them
The Mrkr demo is a fully populated dashboard on real demo traffic, including per-page performance. No signup, no card, cookieless by default.
Open the live demoWhy does the 75th percentile matter so much?
Because averages let you ignore the people having the worst time. A page that loads in 1.2 seconds for three quarters of visitors and 9 seconds for the rest averages to about 3.2 seconds, which sounds mild. In reality one visitor in four stares at a blank screen for nine seconds and most of them leave. To pass at the 75th percentile, three out of four visits have to be good, which forces you to care about older phones, weak connections and visitors far from your servers.
One honest caveat on ranking: Core Web Vitals are a real signal but a light one, and no amount of speed will outrank a page that answers the query better than yours. Fix them because slow pages lose people before they read anything, and because a layout that jumps makes visitors click the wrong thing. The ranking effect is a bonus.
Questions, answered.
Keep reading
- Core Web Vitals monitoring in Mrkr
Continuous field measurement from your real visitors, per page and per device.
- Core Web Vitals, defined
The three metrics, the thresholds, and how the assessment actually works.
- Analytics without cookies
Removing the consent banner removes one of the heaviest scripts on the page.
- Bounce rate calculator
The metric slow pages ruin first, and why the number is often misread.
- Conversion rate calculator
Put a number on what the visitors you lose to a slow page were worth.
- All free Mrkr tools
Browser-only calculators and builders, no signup on any of them.
Your first visitor is already here.
Drop in the script and watch them land. It takes about a minute.