Website Carbon™ provides an estimate of the carbon emissions associated with a single load of a webpage.
When you add a URL to the calculator and click calculate a number of things happen.
Firstly it loads the webpage in the same way a human using a browser would. This gives the calculator it’s first data point, the data transfer in KB. This is the amount of data it takes for a page to load.
The calculator uses this amount of data transfer and estimates the amount of emissions based on V4 of the Sustainable Web Design Model.
The model uses the global average grid intensity to estimate the emissions associated with hosting. It checks the Green Web Foundation Directory to verify if hosting uses renewable energy.
With those checks done, Website Carbon™ then adds in an amount of emissions associated with the transmission network. This is based on the amount of electricity required to transmit the data from the host to your device.
Finally, the calculator adds the emissions associated with you loading the page. As with hosting, this calculation uses a global average grid intensity. You should note that the type of device is not taken into consideration.
Based on the above factors, the webpage will be given an amount of CO2e emissions in grams and an associated rating.
Update to V4 methodology
Website Carbon™ updated to the latest version (V4) of the Sustainable Web Design Model (SWDM) on 14/07/25.
V4 represents a major change from previous models. Most importantly there were more comprehensive datasets available when updating the model. These gave more clarity to the total energy usage of the internet and allowed more granular analysis to be undertaken.
In particular the team updating the model had more data about:
- Operational and embodied emissions from the internet.
- The energy usage of the data centres, networks and user devices.
- Data transfer and carbon intensity.
The full technical breakdown of how V4 of the SWDM model is calculated is here on the Sustainable Web Design site. This Green Web Foundation post covers some of the key differences between V3 and V4 of the model. You can read Wholegrain’s take on the changes here.
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
If you have further questions, please check our FAQ.