How does it work?

Please note that a more accurate calculator, version 3 to be released in the near future (mid 2023). We support our industry peers’ work on researching and developing an up-to-date unified approach.

Whilst the efficiency of some of the web technologies are increasing, there’s a long way to go in reducing the carbon footprint caused by websites.

The approach of this carbon calculator is to make the most realistic estimates that are practical. Measuring a website’s carbon emissions is a challenge but we can get a fairly good estimate by focusing on the data transfer that happens when a website is loaded and energy intensity of internet data, data centre energy source and carbon intensity of electricity.

Data transfer on page load

The data transfer that happens when a website is loaded and amount of data directly correlates with the energy used. Carbon calculator measures the data sent across the wire when the web page is loaded and multiply it by the energy usage data available. There’s an adjustment for repeat visits by the same users to your website as they may have these files cached on their devices.

Web data energy intensity

The data centre, telecom networks, and the end user’s computer or mobile device all need energy. This does vary for each website and each visitor, so we use an average number.

Data centre energy source

Additionally, for the data centre energy use, we check the Green Web Foundation database to see if the data centre is using green energy. If so, then we reduce the carbon emissions attributed to that portion of the energy accordingly. In terms of the energy source, it is assumed that all websites use standard grid electricity for the telecoms network and end user, since there is no way at present to determine otherwise.

Carbon intensity of electricity

The International Energy Agency (IEA) is a body that works with governments and industry to shape a secure and sustainable energy future. The carbon intensity of grid electricity is based on the international average as reported by the IEA (475 grams CO2e per kWh) and renewable energy is based on data for wind energy from Ecotricity (33.4 grams CO2e per kWh). Carbon dioxide equivalent or CO2e is a term for describing different greenhouse gases in a common unit.

Google PageSpeed

Google PageSpeed Insights is a tool that allows you to test the speed of a web page quickly and easily. Page speed is an established ranking criterion for Google’s search results. A 90 or higher is regarded as a good score. Below 50 is regarded poor, while a score between 50 and 90 needs improvement.

Pages of a website we test

This open and public tool is designed to be simple and give an approximate idea of website’s carbon emissions. You can test any specific pages of your website using the url.


ClimateCare (2021) Infographic: The Carbon Footprint of the Internet. Available at:

Green Peace (2017) Clicking Clean: Who is winning the race to build a Green Internet? Available at:

Andrae, A.S.G.; Edler, T. On Global Electricity Usage of Communication Technology: Trends to 2030. Challenges (2015), 6, 117-157.

The Green Web Foundation (2021) The Green Hosting Directory. Available at:

BBC (2020) Why your internet habits are not as clean as you think. Available at:

Nature (2018) How to stop data centres from gobbling up the world’s electricity. Available at:

Our World in Data (2020) CO₂ and Greenhouse Gas Emissions. Available at: