Website carbon: measure the carbon footprint of your site, and simply reduce it Green

Every page you load consumes electricity. On a server, across the network, and above all on your visitor's device. "Website carbon" is that consumption translated into grams of CO₂. Good news: the levers that bring it down are the same ones that make a site fast.

What exactly is website carbon?

A web page doesn't display itself. Data leaves a data centre, crosses a network, and arrives on a phone or a computer, which then has to decode it and render it. Every step consumes energy, and that energy has a carbon intensity.

Website carbon estimates that energy and converts it into grams of CO₂ equivalent per page view. It's an estimate, not a sensor reading. But it's robust enough to compare two versions of the same page and see whether you're making progress.

The reference model is called the Sustainable Web Design Model (SWDM). Version 4 splits the system into three segments: data centres (22% of the energy), networks (24%) and user devices (54%). Worth remembering: more than half the impact happens on your visitor's device, not in your hosting.

Four figures to put it in perspective

1. 0.36 g | CO₂e on average per page view (SWDM v4 model)
2. 2.6 MB | median weight of a mobile page in 2025
3. +203% | growth in mobile page weight since 2015
4. 4.4% | digital's share of France's emissions (ADEME)

A sense of scale

A site with 10,000 page views a month emits roughly 43 kg of CO₂e a year at the average rate. That's not a Brussels–New York return flight. But multiply it by the millions of sites online, add the manufacturing of devices 60% of a device's environmental impact comes from its production, not its use and the total becomes significant. In France, ADEME estimates that the footprint of digital could triple by 2050 if nothing changes.

Why sites keep getting heavier

The median weight of a mobile page has tripled in ten years. Here's where the bytes go today:

Median weight by resource type on mobile (Web Almanac 2025)

Resource | Median weight (mobile) Images | 911 KB
JavaScript | 632 KB
Fonts | 122 KB
CSS | 77 KB
HTML | 22 KB

Two culprits stand out: images and JavaScript. On a median desktop page, roughly 280 KB of JavaScript is downloaded and then never executed. That's dead weight, paid for by your visitors' battery and data plan.

Eight simple actions, in order of payoff

1. Compress and resize your images
   This is the first lever, by a wide margin. Serve your images as AVIF or WebP, at the right dimensions (not a 3000 px image displayed in a 600 px block), with srcset to match the size to the screen. On a typical brochure site, you'll often cut the weight of your visuals by three or four with no visible difference.
2. Turn on lazy loading
   A simple loading="lazy" on images and iframes below the fold. What isn't seen isn't downloaded. Just don't apply it to the page's main image, which needs to stay a priority.
3. Clear out the third-party scripts
   Live chat, ad pixels, interactive maps, duplicate analytics tools, an oversized cookie banner: list everything that loads and ask yourself what actually earns its place. Every third-party script adds requests, weight and often connections to other domains. A lightweight, privacy-friendly analytics tool weighs a few kilobytes where others weigh hundreds.
4. Limit your fonts and self-host them
   Two font families maximum, two or three weights, in WOFF2, served from your own domain with font-display: swap. And ask yourself the question: a system font stack costs zero bytes and displays instantly.
5. Move to a host running on renewable energy
   It won't reduce the weight of your pages, but it does reduce the carbon intensity of the server side. The Green Web Foundation directory lets you check in one search whether your current host is listed. Check where your servers sit, too: the closer to your visitors, the less network travel.
6. Cache properly
   Long cache headers on your static assets, a CDN if your audience is international, and Brotli compression enabled server-side. A returning visitor should re-download almost nothing.
7. Keep video under control
   Video is the heaviest item on the web. No autoplay, no decorative background video on loop, a clickable thumbnail rather than an embedded player loaded by default. And serve the resolution that matches the display size, not 4K in a 400 px frame.
8. Delete pages
   The most overlooked lever. Obsolete pages, articles nobody reads, 12 MB PDFs dating from 2019: all of it is stored, backed up, indexed. An annual content clear-out lightens the site, improves navigation and concentrates your SEO authority on the pages that matter.