How the estimate works
Loading a web page uses electricity in data centres, in the networks that carry the data and on the visitor's device. We measure the bytes your page transfers in an uncached mobile load with Google Lighthouse, then apply the Sustainable Web Design model version 4, the same model used by the open-source CO2.js library. It converts data transfer into energy (about 0.3 kWh per gigabyte across all segments) and energy into emissions using a global average grid intensity.
The result is shown as roughly X grams of CO2e per view, plus a figure per 10,000 views. The formula is printed in the report so you can reproduce it.
What the figure does and does not tell you
Treat the number as an order of magnitude. It ignores caching for repeat visitors, the actual energy mix of your host, the type of device and how long people stay on the page. A host running on renewable energy, for example, would lower the real figure. There is also academic debate about attributing network energy by bytes at all.
What the estimate is reliable for is comparison: the same page before and after an optimisation, or your page next to a competitor's. Because it is driven by bytes transferred, every reduction in page weight reduces the estimate proportionally.
Making your site lighter
These steps also make the site faster and often improve privacy, because many heavy third-party scripts are also trackers.
- Compress and resize images and serve them as WebP or AVIF.
- Remove unused scripts, plugins, sliders and autoplay videos.
- Self-host one or two font weights instead of loading whole families from third parties.
- Cache pages and assets so repeat visitors download less.
- Choose a host that publishes its energy sourcing, if that matters to your firm.
Comparing pages and sites
Run the calculator on your homepage and on a few heavy pages, such as a page with a video or a gallery, to see where most bytes go. You can also compare your site with similar firms. Because every figure comes from the same model and settings, the differences are meaningful even though the absolute numbers are estimates. The page weight checker shows the detailed breakdown behind the figure.