How to measure and reduce the CO2 footprint of your website/app.

The CO2 footprint of a website can be calculated in a few steps, with reasonable accuracy depending on the data shared by the hosting provider and the available traffic data. Once you know the CO2 emissions related to your website, simple and measurable actions to reduce emissions can be taken.

I. The theory : understand the methodology behind CO2 footprint calculation for a website/app

1. Calculate server emissions /GB

Identify the hosting provider and server location where your website is hosted. Different hosting providers have varying levels of energy efficiency and can use a different energy mix (nuclear, renewable, gas, coal, etc) to power their data centers.

Contact your hosting provider or review the documentation to obtain information about the kCO2e emitted per each GB transferred. This is the emission factor that you will use in your calculation.

If your hosting provider does not communicate on this information… it is a problem and you should really consider working with a different one, but you can still go on and use average CO2 emission data for servers.

Example, for a hosting provider : AWS EU west 3 datacenter’s emissions are of 0.000058kgCO2e/GB, so 58gCO2e/mb.

2. Conduct website resource analysis for each page to determine the data volume transferred/page

Each time a user visits a page on your website, website files are transferred through the internet for the page to load on the user’s device.

It is necessary to analyze the resources your website uses, such as images, scripts, and videos.

For each page you need to calculate the size in kb of the data transferred to load the content of your page.

Example, for a page :

Font : 90 kb

Images : 550kb

Video : 840 kb

Data transferred : 1.480kb = 1.48mb

If you have more precise data of user behavior on your web page, you can use it to improve the accuracy of your calculation. For example, if only 20% of the users choose to play the video, you can take that into account in your calculation (90+550+840×0.4=976kb).

3. Multiply by web traffic for each page to calculate total CO2 emissions associated with the page

Using your traffic analysis tool (Google Analytics for example), multiply the volume transferred/page by the traffic of each page, on the reporting period.
Example, for a page :

Hosting provider emission factor : 26 g CO2e per MB.

Data volume of the page : 976kb

Visits of the page in 2022 : 15.264

CO2 footprint for one visit of the page = 26×0.976 = 25,4g CO2e/visit

CO2 footprint for the page in 2022 = 15.264×0.0254 = 387,7 kg CO2e

II. In practice : tools to calculate your website’s footprint easily

The methodology explained in I. can be implemented quickly and easily using ready-made tools.

Firstly, your hosting provider may have a CO2 footprint calculation tool that gives you the CO2 footprint of your website on a given period, taking into account the real traffic and data volume transferred by your website. This is by far the easiest and most precise option. This information can also be available on your dashboard or on invoices.

AWS for example offers a carbon footprint tool to its clients.

If you cannot get that information from your provider, free tools like Website Carbon enable you to calculate easily the footprint of a web page.

III. How to reduce the CO2 footprint of your website?

🗺️Have the simplest user experience (UX) possible

Users should be able to find the content they want and perform the actions that they need to take in the easiest manner possible. This is good for your business, but also for the CO2 footprint of your website since it reduces the amount of pages visited “by mistake” by users when they are looking for something.

👸“This isn’t Versailles!” : optimise images, videos and fonts

Videos and images are often the highest contributors to the CO2 footprint of a web page. Use them only when necessary.

Compressing images with tools such as TinyPNG or ShortPixel will reduce the data transfer volume without any noticeable change in quality.

For videos, avoid at all cost videos playing automatically (no one likes this, anyway). Make the default quality HD/720p or lower (not full HD or 4K).

As for fonts : use standard system fonts (Helvetica or Arial, for example), common fonts (Google Fonts like Roboto) or if you use custom fonts, opt for modern web font file formats like WOFF and WOFF2, which use higher compression methods compared to TTF, OFT and SVG file formats.

Reducing the volume of the images, videos and fonts loaded will reduce the CO2 emissions/visit while also reducing loading time for your page, which will also boost your SEO and improve UX.

🌱Code green

Keep code as clean and simple as possible. Avoid Javascript (adds weight to the web page). If you use a CMS manager such as WordPress, Wix or Shopify : avoid unnecessary plugins and pay attention to the size of the plugins that you use.

Using server caching can also help reducing the CO2 emissions/visit for users coming back to your website.

🔌Choose your hosting provider wisely

Your website’s footprint depends A LOT on your hosting provider. You should look at two main factors :

• The energy mix used by the hosting company : a provider based in a region where the energy grid is low carbon (mostly from renewable or nuclear) or with a trusted green energy purchase commitment (100% certified green energy purchase) will lower considerably the g CO2e/mb.
• The energy efficiency of the hosting company : you should look for the Power Usage Efficiency, or PUE, of your provider. The PUE is calculated by dividing the amount of energy entering the data centre by the amount of energy used to run the computers inside it. The PUE shows the amount of energy being wasted on non-computational activities such as cooling. The average PUE for a data centre is around 1.67, meaning that for every 1.67 watts going into the data centre, 1 watt is being used to power the computing systems. The closer to 1 the PUE, the more energy-efficient the data center is.

The Green Web Directory provides a list of verified green hosting providers by country in all the countries that appear in green in the following map.

Among all the possible reduction actions, you can evaluate the impact of each action, in particular if you need to prioritize. For example, compressing images on your home page to reduce their total size from 600kb to 400kb, with a 180.000 total annual traffic and an emission factor of 15g CO2e/mb (provider data) will result in a 540kgCO2e reduction in your website’s total footprint. Users will not notice any quality difference but will probably appreciate the quicker loading time. So what are you waiting for?

Sources & useful links :

The Website Carbon Calculator provides a reliable calculator, tutorials and tips to improve your website’s CO2 footprint.

The Green Web Directory lists green hosting providers and can help you in choosing low carbon hosting.

TinyPNG to compress images

ShortPixel to compress images