Lighthouse
29 quoted answers, each with its source
The short answer
Lighthouse is an open-source, automated tool for improving web page quality. Its performance score is a weighted average of metric scores from a log-normal distribution derived from HTTP Archive data. As of Lighthouse v6 in 2019-09-19, desktop runs use specific desktop scoring instead of mobile-based curves, and as of Lighthouse 10 in 2023-02-09, the Time to Interactive metric was removed with its weight shifted to Cumulative Layout Shift, which now accounts for 25% of the score. A perfect score of 100 is not expected, and Google does not use the X/100 Lighthouse score for search ranking; it evaluates Core Web Vitals separately.
Every part of this comes from the quotes below. How we write these.
What the rules actually say19
Quoted straight from the people who make or enforce the rules.
- Lighthouse's Agentic Browsing category does not have a weighted average score from 0 to 100.
- Other Lighthouse categories have a weighted average score from 0 to 100.
1 quote from 1 source
Unlike other Lighthouse categories, the Agentic Browsing category does not have a weighted average score from 0 to 100. Because the standards for the agentic web are still emerging, the current focus is to gather data and provide actionable signals rather than a definitive ranking.
Chrome for Developers' Lighthouse agentic browsing scoring documentation states that…
- the Agentic Browsing category is experimental.
- WebMCP support is experimental.
- testing the Agentic Browsing category requires Chrome 150 or later.
1 quote from 1 source
Note: The Agentic Browsing category and WebMCP support are experimental and based on proposed standards. Testing this category requires Chrome 150 or later, and WebMCP audits require registering for the WebMCP origin trial.
Google Search Central's page experience documentation states that trying to get a perfect Lighthouse score just for SEO reasons may not be the best use of your time.
1 quote from 1 source
These scores are meant to help you to improve your site for your users overall, and trying to get a perfect score just for SEO reasons may not be the best use of your time.
Chrome for Developers' Lighthouse overview documentation describes Lighthouse as…
- an open-source tool.
- an automated tool.
- a tool to help you improve the quality of web pages.
1 quote from 1 source
Lighthouse is an open-source, automated tool to help you improve the quality of web pages.
web.dev's Web Vitals article states that tools like Lighthouse…
- load pages in a simulated environment without a user.
- cannot measure INP because there is no user input.
1 quote from 1 source
Note: Tools like Lighthouse that load pages in a simulated environment without a user cannot measure INP, as there is no user input. However, the Total Blocking Time (TBT) metric is lab-measurable and is a proxy for INP.
- Web Vitals | Articles | web.dev web.dev
The Lighthouse 10 announcement on the Chrome for Developers blog states that Cumulative Layout Shift will now account for 25% of the overall performance score.
1 quote from 1 source
The venerable Time To Interactive (TTI) metric is being removed in Lighthouse 10, concluding the deprecation process started in Lighthouse 8. TTI's 10% score weight is shifting to Cumulative Layout Shift (CLS), which will now account for 25% of the overall performance score.
Chrome for Developers' Lighthouse performance scoring documentation states that the Lighthouse Performance score is a weighted average of the metric scores.
1 quote from 1 source
The Performance score is a weighted average of the metric scores.
The Lighthouse scoring distribution is…
- a log-normal distribution.
- derived from the performance metrics of real website performance data on HTTP Archive.
1 quote from 1 source
The scoring distribution is a log-normal distribution derived from the performance metrics of real website performance data on HTTP Archive.
Chrome for Developers' Lighthouse performance scoring documentation states that a perfect Lighthouse score of 100 is…
- extremely challenging to achieve.
- not expected.
1 quote from 1 source
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.
Chrome for Developers' Lighthouse performance scoring documentation states that a lot of the variability in the overall Performance score and metric values is not due to Lighthouse but to changes in underlying conditions.
1 quote from 1 source
A lot of the variability in your overall Performance score and metric values is not due to Lighthouse. When your Performance score fluctuates it's usually because of changes in underlying conditions.
All Lighthouse score curves prior to v6 were based on mobile performance data.
1 quote from 1 source
Prior to Lighthouse v6, all score curves were based on mobile performance data, however a desktop Lighthouse run would use that. In practice, this led to artificially inflated desktop scores. Lighthouse v6 fixed this bug by using specific desktop scoring.
Measured finding4
Something someone actually measured, with the method attached.
- The Lighthouse 10 announcement on the Chrome for Developers blog reports an analysis of 13 million page loads in the latest HTTP Archive run.
- The analysis reported by the Lighthouse 10 announcement on the Chrome for Developers blog states that 90% of the 13 million page loads would see an improvement in their Lighthouse performance score.
1 quote from 1 source
In an analysis of 13 million page loads in the latest HTTP Archive run, 90% of those pages would see an improvement in their Lighthouse performance score, with 50% of them seeing a performance improvement of more than 5 points.
Lighthouse's 'Milliseconds make millions' case study states that the research team…
- studied 37 leading European and American brand sites.
- collected data on over 30 million user sessions.
1 quote from 1 source
The research team studied 37 leading European and American brand sites, collecting data on over 30 million user sessions.
Practical effect2
What is observed to happen in practice, as distinct from what is documented.
Search Engine Journal reports that Google's John Mueller wrote that Google…
- does not use the X/100 Lighthouse score for search.
- uses the Core Web Vitals separately from the X/100 Lighthouse score.
2 quotes from 1 source
Google's John Mueller responded to one such question in the r/SEO forum on Reddit this week.
Google doesn't use the X/100 lighthouse score for search, we use the core web vitals separately (lcp, cls, fid). I think you can get those from Lighthouse too, but there are lots of other tools that also show them.
Changed on4
A dated change. This is where advice quietly stops being true.
The Lighthouse 10 announcement on the Chrome for Developers blog states that the…
- Time To Interactive metric is being removed in Lighthouse 10.
- 10% score weight of the Time To Interactive metric is shifting to Cumulative Layout Shift.
1 quote from 1 source
The venerable Time To Interactive (TTI) metric is being removed in Lighthouse 10, concluding the deprecation process started in Lighthouse 8. TTI's 10% score weight is shifting to Cumulative Layout Shift (CLS), which will now account for 25% of the overall performance score.
- Mobile-based score curves before Lighthouse v6 artificially inflated desktop scores.
- Lighthouse v6 fixed this by using specific desktop scoring.
1 quote from 1 source
Prior to Lighthouse v6, all score curves were based on mobile performance data, however a desktop Lighthouse run would use that. In practice, this led to artificially inflated desktop scores. Lighthouse v6 fixed this bug by using specific desktop scoring.