Guide to Using the Lighthouse Scoring Calculator

Need help understanding how different website loading speed metrics will affect your Lighthouse score?

The Lighthouse Scoring Calculator is a web tool provided by Google Chrome to help developers understand how Lighthouse generates performance scores for web pages. Lighthouse is an open-source tool used for auditing web pages, focusing on performance, accessibility, SEO, and best practices.

Why Use the Lighthouse Scoring Calculator?

• Helps developers understand how performance scores are computed.
• Aids in prioritizing optimizations for better Lighthouse results.
• Useful for debugging and experimenting without running full Lighthouse audits.

How It Works

The calculator allows you to manually input individual audit scores and see how they contribute to the overall Performance Score.

1. Access the Tool: Visit Lighthouse Scoring Calculator.

2. Select Lighthouse Version: Choose the Lighthouse version you want to use. Different versions may have slight changes in score weighting.

3. Enter Audit Scores: The calculator presents various performance metrics, such as:

• First Contentful Paint (FCP)
• Largest Contentful Paint (LCP)
• Total Blocking Time (TBT)
• Cumulative Layout Shift (CLS)
• Speed Index (SI)

4. Adjust each metric’s value to see its impact on the overall score.

Understand Weighting

Each metric has a different influence on the final score. For example:

• TBT and LCP are heavily weighted (impacting the score more than others).
• CLS has a lower weight but still affects the score significantly.

Observe Score Changes

As you modify individual metric values, the overall Performance Score updates dynamically.

• Optimize Based on Insights. Use the calculator to identify weak areas in your website’s performance.
• Improve the most impactful metrics to boost the overall score.

What Is a Good Lighthouse Performance Score?

To provide a good user experience, sites should strive to have a good score (90-100). A "perfect" score of 100 is extremely challenging to achieve and not expected. For example, taking a score from 99 to 100 needs about the same amount of metric improvement that would take a 90 to 94.

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