Sustainability: page carbon estimates

Each public page on this site shows an estimate of the carbon emitted when it is viewed once. The figures are estimates, not measurements, and are recalculated every time the site is built. Across this site's pages the estimate ranges from 0.020 to 0.026 g CO2e per view, with an average of 0.020 g.

Annexure: method and limits

Model. Sustainable Web Design model, version 4. It converts the data a page transfers into energy and then into CO2e, across three segments: data centres, networks and the visitor's device, each with operational and embodied (manufacturing) energy.

Constants. 0.300 kWh per GB in total (data centres 0.067, networks 0.072, devices 0.161) and a global grid average of 494 g CO2e per kWh, which gives about 148.2 g CO2e per GB transferred.

Page size. The compressed HTML plus every same-site stylesheet, script, font and image the page references, measured from the built files. Text files are counted at gzip size; fonts and images at file size. Because every font file the page lists is counted, whether or not a browser needs all of them, the figures lean high.

Assumptions. Every view is treated as a first visit with nothing cached. No credit is taken for renewable energy at the host, so the figures are conservative. The pages use no analytics, tracking scripts, video or third-party fonts.

Limits. This is an estimate based on an industry model and average figures. It is not an audit or certification, and it does not claim the site is carbon neutral. Other tools give different numbers for the same page. Source: sustainablewebdesign.org.