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Estimate an overall performance score from core web metrics.
What this tool helps you answer
Use this tool before running Lighthouse or commissioning a full performance audit. Enter the Core Web Vitals you have measured (or expect) and model how different device and network conditions change the simulated overall score, so you can prioritize optimization work on the factors that will move the needle for your worst-served users.
The score is a directional model. It reflects how the input metrics interact under the selected profile, not a replacement for measured field data. Use it to prioritize, then validate with Lighthouse or CrUX.
Next step
Estimate an overall performance score from core web metrics.
Editorial review
This page combines the live tool, input guidance, worked examples, and operating limits so Website Performance Simulator stays useful even before users interact with the calculator.
Reviewed by Klartext Tools against the current Website Performance Simulator workflow on 2026-02-24.
Last updated:
Use with judgment
Page scope
Good Core Web Vitals on a wired connection with a high-end device: a baseline for a fast-loading marketing page.
High performance score: useful as a reference point before applying mobile throttling.
Switch to Mobile 3G and Low-end device after loading to see how score degrades for the same metrics.
A page with a heavy hero image and slow server response, tested on a mid-range device with 3G throttling.
Low performance score: shows how LCP above 4 seconds drives the most degradation under constrained conditions.
Reduce LCP to 2.5s while keeping other inputs fixed to isolate the LCP contribution to the score improvement.
Enter the Core Web Vitals from a recent Lighthouse run or from your CrUX field data. Use real measurements rather than default values to get directionally useful output.
Enter FCP and LCP in seconds, INP in milliseconds, and CLS as a decimal.
Select the network profile that best represents your target audience. Lab/Wired for desktop benchmarks, Mobile 4G or 3G for realistic field conditions.
Select the CPU profile that matches the device tier you care about most.
Run the simulation and note the headline score and the per-metric status indicators.
Change one input at a time to identify which metric contributes most to score degradation under the selected profile.
Try a well-optimized scenario and a degraded mobile scenario to understand how the scoring model weights each metric.
Good Core Web Vitals on a wired connection with a high-end device: a baseline for a fast-loading marketing page.
Sample inputs
Sample outcome: High performance score: useful as a reference point before applying mobile throttling.
Switch to Mobile 3G and Low-end device after loading to see how score degrades for the same metrics.
A page with a heavy hero image and slow server response, tested on a mid-range device with 3G throttling.
Sample inputs
Sample outcome: Low performance score: shows how LCP above 4 seconds drives the most degradation under constrained conditions.
Reduce LCP to 2.5s while keeping other inputs fixed to isolate the LCP contribution to the score improvement.
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