CO₂ Emissions
Every page PerfLeaf scans includes an estimated CO₂ footprint. This helps you understand the environmental cost of your website and track improvements over time.
How it’s calculated
Section titled “How it’s calculated”PerfLeaf uses co2.js (by the Green Web Foundation) to estimate the CO₂ emissions of each page load. The estimate is based on:
- Total data transferred — the combined size of all resources loaded by the page.
- Energy intensity model — co2.js converts bytes transferred to kilowatt-hours using a standardised energy model.
- Carbon intensity — the energy is converted to grams of CO₂ using average grid carbon intensity figures.
The result is expressed in grams of CO₂ per page load (gCO₂).
Report view
Section titled “Report view”Site-level
Section titled “Site-level”The CO₂ Emissions card in the site report shows:
- Total CO₂ — sum of all pages in the scan
- Average CO₂ per page — mean across all crawled pages
Page-level
Section titled “Page-level”The page report shows the exact gCO₂ figure for that specific URL.
Scoring
Section titled “Scoring”CO₂ is one of the six Performance Score categories. Pages with higher emissions receive a lower CO₂ score.
Tracking over time
Section titled “Tracking over time”Use the Trends view to track your site’s average CO₂ per page over time. A lighter, faster site is also a greener one.
Reducing emissions
Section titled “Reducing emissions”Since CO₂ is directly related to page weight, the same fixes that improve your Page Assets score will also reduce emissions:
- Compress and convert images to WebP/AVIF
- Minify and tree-shake JavaScript and CSS
- Remove unused third-party scripts
- Enable CDN caching to reduce repeat data transfers
- Use a green hosting provider