How does it work?

Calculating the carbon emissions of website is somewhat of a challenge, but we have been working for many years to develop and refine a methodology for this purpose. Our hope is that this will help raise awareness and encourage more eco-friendly approaches to be adopted throughout the web design industry.

Having developed the original website carbon calculator several years ago and iterated the methodology, we now aim to encourage the adoption of a standardised approach to website carbon calculations. We hope that this will provide more clarity and understanding, as well as help to prevent greenwashing. For version 3 of our methodology, we have therefore partnered with industry peers to conduct the research and develop an updated coordinated approach.


To calculate the energy and emission of a web page we use the following data points:

  1. Data transfer over the wire
  2. Energy intensity of web data
  3. Energy source used by the data centre
  4. Carbon intensity of electricity
  5. Website traffic

Data transfer over the wire

When a website is loaded, the energy used is roughly proportional to the amount of data transferred.  We measure the data transferred over the wire when a web page is loaded, and multiply that by the energy usage data that we have.  We also make an adjustment for repeat visitors who may have website assets cached on their devices.

Energy intensity of web data

Energy is used at the data center, telecoms networks and by the end user’s computer or mobile device.  Of course, this varies for every website and every visitor and so we use an average figure

Energy source used by the data center

To gauge the energy source, we assume that all websites use standard grid electricity for the telecoms network and end user, since we have no way to determine otherwise.  For the data center energy use, we check The Green Web Foundation (TGWF) database to see if the data center is using green energy.  If so, then we reduce the carbon emissions attributed to that portion of the energy accordingly. Please note that The Green Web Foundation database includes data centres that purchase standard grid electricity but offset their emissions.  For the purposes of this calculator, we treat them all the same.

Carbon intensity of electricity

The carbon intensity of grid electricity is based on the international average for grid electricity. We use a lower carbon factor for renewable electricity when used in the data center.

Website traffic

When we put all of this information together, we get a pretty good idea of the emissions associated with an average user visiting any given website.  Multiply carbon per page view up by the typical number of annual page views and we can estimate the total annual CO2 emissions.

Which pages do we test?

The public version of the tool is designed to be simple and give a rough idea of website efficiency and so we only test the single url that you enter into the form, but you can test as many separate urls as you like.

Any website can be tested but only sites that comply with the following guidelines are included in our ranking tables:

  • Can be accessed by the public through a standard web browser
  • Do not require login
  • Allow search engines
  • Contain unique content aimed at human visitors – this excludes holding pages, error pages, server notification pages, demo pages or pages that are generally useless (this is highly subjective)
  • Are free from illegal or explicit content

Further information

For full details of the version 3 methodology, please read more at

If you have further questions, please check our FAQ.


We're just loading your results

Try testing again