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A millisecond might seem like such a short amount of time that it’s inconsequential. But for every 100 milliseconds a website takes to load, the conversion rate can drop by 7%.

If you want your agency to not only survive, but to thrive, you need to make sure your clients’ websites are as optimized and healthy as possible—performing to the best of their ability as users try to navigate. That’s where Google Lighthouse metrics come in.

In our guide to Google Lighthouse metrics, you’ll learn about the website metrics and performance data you should be tracking to help your clients’ sites perform better. We’ll explain concepts like Total Blocking Time, Largest Contentful Paint, and others, sharing what’s considered a “good” score in each of these categories and how you will use these metrics to improve.

Let’s get started breaking down the Google Lighthouse website metrics you should be analyzing to help improve your clients’ website performance.

• What Is Google Lighthouse?
• What You Need To Know About The Google Lighthouse Scoring System
• Google Lighthouse Performance Metrics Explained
• 6 Key Google Lighthouse Metrics to Track
• How To Use Google Lighthouse
• How To Analyze the Lighthouse Performance Report
• Takeaway

What Is Google Lighthouse?

Google Lighthouse is an open-source, automated tool that measures the quality of a web page. Use Google Lighthouse audits to gauge a website’s performance and audit progressive web apps, getting personalized insights on how to improve user experience.

With this tool, your agency learns what technical changes should be made to help your clients’ websites load more quickly, be more accessible, and rank higher in search engine results pages (SERPs)—which will all result in more people finding the website, and those people staying on the site for longer once they do.

Google Lighthouse works for any web page—public or private. It’s free and easy to use, and offers helpful metrics (Core Web Vitals) to assess site performance and figure out which areas need to improve.

Google Lighthouse vs. Core Web Vitals

Google Lighthouse uses three specific metrics to measure website performance. These Core Web Vitals are:

• Largest Contentful Paint. This metric uses LCP data to measure how a website visitor perceives the page loading experience (how quickly the largest image or text block shows up).
• Total Blocking Time. Think of Total Blocking Time as a website’s responsiveness to user input. It represents the time between ​​First Contentful Paint and Time to Interactive.
• Cumulative Layout Shift. The third and final Core Web Vital looks at how web page elements move, to measure how visually stable the page appears.

When you receive the Google Lighthouse performance score, these vitals will be included. Together, they make up 70% of the weighted performance score.

What You Need To Know About The Google Lighthouse Scoring System

Running an audit on Google Lighthouse will result in an overall performance score. Here’s how the scoring system works.

What Is the Lighthouse Performance Score?

Google Lighthouse uses a scoring system to evaluate a website and provide a rating from 0 to 100. To create this rating, the tool considers different web performance metrics or categories and adds them up:

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

The higher the score, the healthier and more effective the website. A good Google Lighthouse score is anything above 90.

Why Lighthouse Scores Fluctuate

You might notice that a client’s Lighthouse report score changes over time. Google might give you a different number each time you test the site. This is normal—don’t worry! There are several factors that can affect the score on any given day, including:

• Server response times
• Network conditions
• Browser extensions
• Web page ads
• Page customizations like AB tests
• Network data routes
• The length, time, and location of the test
• How powerful the CPU is
• Different Lighthouse versions or settings

It’s natural to see some variability in a Lighthouse score.

Google Lighthouse Desktop vs. Mobile

You might also notice that you receive a different performance rating if you run the test on the desktop vs. the mobile version of your client’s website. Again, don’t panic! It’s common for mobile scores to come back lower. This might be because the mobile device has a slower processor or weaker connection than a desktop computer.

Google Lighthouse Performance Metrics Explained

There are several different metrics that go into the total Google Lighthouse score. It can be confusing to think about how a score is calculated. Factors like accessibility, performance, and SEO all play a role. Take a look at the individual website performance metrics Google uses to build the overall final score.

How Is The Lighthouse Performance Score Calculated?

Google calculates the Lighthouse performance score using a weighted average of different metrics. Each of these metrics indicates a different aspect of the web application page load speed and responsiveness. Google gives each metric a weight. Then you receive the final score, which will be a number between 0-100 (the higher, the better).

Performance Audit

The performance audit portion of the overall Google Lighthouse audit indicates how healthy your client’s website performance is. The score is an aggregate based on a number of the page-level metrics we’ve listed above. The closer the score is to 100, the better.