Core Web Vitals Simulator
See how your page might score on Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS). Paste HTML or test a live URL – all analysis runs in your browser.
What is a Core Web Vitals Simulator?
A Core Web Vitals Simulator estimates how your web page might perform on Google's three key user‑experience metrics: Largest Contentful Paint (loading speed), First Input Delay (interactivity), and Cumulative Layout Shift (visual stability). This free tool parses your HTML or fetches a live page and checks for common performance pitfalls – like large images without dimensions, blocking scripts, and missing font display settings – to give you a quick, private preview of potential issues before you run a full Lighthouse audit.
How to Simulate Core Web Vitals
- Paste your HTML source code or enter a URL to test.
- Click "Simulate Web Vitals" or "Fetch & Simulate".
- The tool will estimate LCP, FID, and CLS based on heuristic analysis.
- Review the color‑coded scores and the list of detected issues.
- Use the recommendations to fix the problems before deploying.
Why Use This Free Simulator?
- 100% free: No sign‑up, no limits, no watermarks.
- Private & offline: All analysis happens in your browser.
- Instant feedback: See potential problems without waiting for full audits.
- Actionable issues: Each detection includes a short explanation and suggested fix.
- Mobile friendly: Works perfectly on phones and tablets.
Frequently Asked Questions
It parses the HTML and applies heuristics: LCP is estimated from the largest visible element (image or text block) and its size/load delay; FID from total JavaScript size and main‑thread blocking patterns; CLS from images without dimensions, late‑loading content, and animations. The results are approximate and intended for quick checks.
Yes, completely free. No sign‑up, no limits.
They are estimates, not real measurements. For precise data, use Lighthouse or PageSpeed Insights. This tool helps you catch obvious issues early.
Yes, enter a URL and the tool will fetch its HTML via a public CORS proxy. If that fails, you can paste the HTML directly.
LCP measures loading speed (time to render the largest element), FID measures interactivity (delay before the page responds to user input), and CLS measures visual stability (unexpected layout shifts).
Yes, the tool is fully responsive and works on smartphones and tablets.