How we measure

How do you measure the CO2 emissions of a website?

Our calculations are based on the idea of the Website Carbon API. However, we have rewritten the code for our own purposes and developed our own algorithm to enable a more accurate calculation of CO2 emissions.

We use various data sources to calculate the CO2 emissions of a website in order to provide a good estimate:

• Data transmission and device types
• Energy intensity of web data
• Energy source of the data centre
• Website traffic
• Data transmission

To calculate the amount of data, we analyse the five most visited pages of a website, automatically scrolling through the pages so that all content is loaded (including that which is reloaded). The percentage of the total number of pages visited is then calculated. Using this data, we calculate the individual size of each page and take the weighted average value.

The website size is determined in three different resolutions (these correspond to three different device types: mobile, tablet, desktop). The resulting sizes are weighted according to the device distribution of website visits.

Energy intensity of web data

Energy is consumed in the data centre, in telecommunications networks and by the end user's computer or mobile device. This is of course different for every website and every visitor. Therefore websitecarbon.com uses the average.

The figures used for 2017 come from the report On Global Electricity Usage of Communication Technology: Trends to 2030 by Anders Andrae and Tomas Edler, reduced by the production energy as this is not relevant for this calculator. The total amount of energy consumed is then divided by the total annual data transmission via the internet, which is described in an article in Nature magazine: How to stop data centres gobbling up the world's electricity. This results in a figure of approx. 1.8 kilowatt hours per gigabyte.

Energy source used by the data centre

For the data centre's energy consumption, we check the Green Web Foundation (GWF) database to determine whether the data centre uses green energy. If so, the carbon emissions are reduced accordingly. Of course, the GWF database is not 100% perfect and includes data centres that buy standard grid power but offset their emissions. However, for the purpose of the calculation, all are treated equally.

Website traffic

When all this information is collated, we know the level of emissions associated with an average user visiting a particular website.

If the carbon per page view is multiplied by the number of monthly page views, the monthly CO2 emissions can be measured.