Updating Website Carbon to V4 of the Sustainable Web Design Model

Wholegrain has a three part mission statement.

We aim to create the best websites in the world.

We use our business as a force for good.

We work to accelerate the shift to an Internet that’s good for people and planet.

This mission runs through our organisation like a stick of rock. Sometimes it’s a balancing act. At times we are focussed more on one element than another. 

Website Carbon is one of the ways we aim to shift towards a better internet. Since its launch in August 2018 the tool has been used to test over 1 million individual webpages, for free. For many, using the tool has been a stepping stone into larger actions within digital sustainability. It certainly was for me and many of my colleagues at Wholegrain.

We take the stewardship of Website Carbon very seriously as a result. It means that making any changes to the site takes time and care. The last major update came in November 2023 when we added a rating system to the tool. The ratings give more context to the results and allow for better benchmarking.

Since then the calculator has remained stable but relatively untouched, until today.

On the 14th July 2025 we updated the calculator to the V4 of the Sustainable Web Design Methodology (SWDM). V4 is the latest version of the model. The process of updating the calculator has been delicate and one we were keen to get right the first time. Any issues that erode trust in the estimates the tool provides need to be avoided.

After extensive testing Website Carbon is now updated. If you have a keen eye you might see some changes to the results it provides.

V4 is a major update to the SWDM and things are quite different under it. 

For one thing you might notice that the estimate your website receives in V4 is lower than in V3. Sites tested using the V4 model tend to have lower carbon estimates per page view than V3. Estimates for your webpages may be up to two thirds lower than in V4.

SWDM uses data transfer as the main proxy for estimating webpage energy use. The core calculation is the total energy used by the internet divided by the total data transfer on the internet. The updated sources used for V4 of the model saw that data transfer has increased two fold. However, total energy use has fallen.

Essentially the internet appears to be operating more efficiently than previously thought. 

Not at all!

Efficiencies in the network will only go so far. The total data transfer on the internet is increasing hugely. Growth in AI and data collection continues at an increasing pace. Efficient, lightweight websites are more important than ever.

Your website’s footprint might represent a tiny fraction of the internet’s overall emissions. But a more efficient website has huge benefits outside of carbon reduction. Lower carbon sites tend to be quicker to load and have better structure. Humans like this, search engines like this, they can lead to higher conversion rates and it’s better for the planet.

When Website Carbon launched 7 years ago it was a pioneering tool. As part of our “improve the internet” remit we offered access to the Website Carbon API endpoint to a handful of users. From today that is no longer the case.

The main tool isn’t affected in any way, but it’s important to be transparent when we make changes like this. Any people or organisations materially affected by the change have already been contacted.

The digital sustainability landscape has changed significantly since 2018. Building a similar tool to Website Carbon is accessible. For instance The Green Web Foundation has an open-source JavaScript library CO2.js. with extensive documentation and support.

The change to API endpoint access isn’t one we have made lightly. Changing the way it can be accessed allows us to put our time and energy into other areas.

Watch this space for more details about where our energy is being redirected.

We’re pioneers in the digital sustainability space and have a duty to the community to continually refine our offerings as knowledge and technology evolves. We’re on a mission, and we make our tools and resources publicly available so others can learn and help mobilise this mission.

If you have questions relating to Website Carbon you can reach out to our [email protected].

If you want to dig deeper into your website’s carbon footprint speak to [email protected] about our Digital Sustainability Consulting.

If you want further reading or to take more action you can subscribe to our Curiously Green Newsletter, visit the Sustainable Wed Design site or download the Digital Declutter toolkit.

Here’s to many more years of website testing.