Google's Core Web Vitals decide whether your pages feel fast or sluggish — and they feed into search rankings too. The Web Vitals Calculator is a free online tool that takes your measured values for LCP, INP, CLS, FCP and TTFB and classifies each one against the current Core Web Vitals thresholds, so you can see at a glance what passes, what needs work, and what is failing.

Try it free: Web Vitals Calculator

What the tool does

The Web Vitals Calculator is a focused developer utility for checking web performance numbers. You paste in the values you measured — from Lighthouse, PageSpeed Insights, Chrome DevTools, or your real-user monitoring — and it classifies each metric as Good, Needs Improvement, or Poor against Google's current thresholds:

  • LCP (Largest Contentful Paint) — how fast the main content loads. Good is 2.5 seconds or less.
  • INP (Interaction to Next Paint) — how quickly the page responds to clicks and taps. Good is 200 milliseconds or less.
  • CLS (Cumulative Layout Shift) — how much the page jumps around while loading. Good is 0.1 or less.
  • FCP (First Contentful Paint) — when the first piece of content appears. Good is 1.8 seconds or less.
  • TTFB (Time to First Byte) — how fast the server responds. Good is 800 milliseconds or less.

The tool runs entirely in your browser. There is no account, no signup, and nothing to install. It is built for quick reviews, debugging notes, code review prep, QA handoffs, and documentation examples.

How to use it

  1. Measure your page first. Run Lighthouse in Chrome DevTools, or check PageSpeed Insights for your URL, and note the LCP, INP, CLS, FCP and TTFB values it reports. Real-user data from the Chrome UX Report works too.
  2. Open the calculator. Go to Web Vitals Calculator in any modern browser.
  3. Enter your values. Type or paste each metric into its field. If you only have some of the metrics, enter the ones you have — you can check a single value just as easily as all five.
  4. Run the check. The tool classifies each value against the current Core Web Vitals thresholds and shows whether it is Good, Needs Improvement, or Poor.
  5. Review the findings. Look at which metrics are failing first. A poor LCP usually points at slow images or render-blocking resources; a poor CLS usually means images or ads without reserved space; a poor INP usually means heavy JavaScript blocking the main thread.
  6. Copy or download the result. Copy the output for a ticket or report, or download a text report to share with your team. Use the reset action to clear the fields and check another page.

Practical tips

  • Fix the worst metric first. One failing metric drags the whole experience down. If LCP is poor and everything else passes, focus on the largest image or the server response time — that single fix moves the needle most.
  • Check mobile and desktop separately. Core Web Vitals thresholds are the same, but the numbers are not. A page that passes on desktop can easily fail on a mid-range phone over a slow connection, so run both and fix the mobile numbers first.
  • Re-check after every change. Performance work is easy to undo by accident — a new script, a bigger hero image, a third-party widget. Paste the fresh numbers into the calculator after each deploy and confirm the classification is still Good before you move on.

Frequently asked questions

Does the Web Vitals Calculator upload my data?

No. The normal workflow runs locally in your browser. Your values never leave your device.

Is this a full replacement for Lighthouse or PageSpeed Insights?

No. The calculator classifies values you already measured — it does not measure your page itself. Use Lighthouse or PageSpeed Insights to get the numbers, then use this tool for a fast classification and a shareable summary.

Can I copy or download the result?

Yes. Run the tool, then copy the output or download a text report for your ticket, pull request, or QA handoff.

Do I need an account to use it?

No. There is no account and no signup. Open the page, enter your values, and get the result.

Privacy note

The Web Vitals Calculator runs 100% in your browser — your measurements and any pasted input never leave your device. There is no server upload, no tracking of your values, and no account needed.

Try it free: Web Vitals Calculator