- 1A page view emits 0.7 to 1.5 g of CO2e once user devices are counted, two to four times what free tools report.
- 2The user's device accounts for 45 to 60% of the digital carbon footprint, and free calculators estimate it poorly.
- 34 priority levers: images, JavaScript, caching with a content delivery network and the choice of hosting provider.
- 4The French RGESN eco-design framework remains voluntary, and the REEN law mainly requires local authorities to adopt a digital sustainability strategy.
When a web page loads, energy is used at 4 points in the chain: the data centre that hosts the site, the network that carries the data to you, the device (smartphone or PC) that displays the page, and the manufacture of all this equipment (its embodied emissions). Online tools estimate the first 2 reasonably well; the last 2, which account for about 60% of the real impact, are either ignored by free calculators or only roughly estimated.
Where does the carbon footprint of a website come from?
4 emission sources. Free calculators estimate the data centre and the network well (40% of the total), and the devices poorly or not at all.
The figures in circulation are therefore often too low. Website Carbon Calculator reports about 0.36 gCO2e per page view for an average page, whereas a calculation that fully includes the user's device and its manufacture gives closer to 0.7 to 1.5 g, which is 2 to 4 times more. Between a light page and a heavy one (about 8 MB for the heaviest 10% of mobile pages, according to HTTP Archive), the difference in emissions easily exceeds a factor of 10.
1Where a website's emissions come from: the user's device matters most
The data centre accounts for 15 to 25% of the impact. The servers that host your site use electricity, as does the cooling of the building, which often uses water too, as in AI data centres. A data centre's efficiency is measured by its PUE (power usage effectiveness): the global average is around 1.6 to 1.7, and the best facilities are below 1.2. The country remains the decisive factor: thanks to France's low-carbon electricity, a server there emits 7 to 8 times less than a server in Germany and 10 to 15 times less than one in Poland.
The 4 emission sources of a website
Click on each source to see how it is calculated.
The network accounts for 10 to 20%. Every page passes through several relays: routers, submarine cables and mobile masts. Wi-Fi uses 3 to 5 times less energy than 4G or 5G to transfer the same amount of data, and 60 to 70% of web traffic now goes through mobile, which increases the network's share.
The user's device accounts for 20 to 30% of the impact in use and 30 to 45% in manufacturing. The heavier your page, the more energy the visitor's device uses to display it, but most of the impact arises upstream: manufacturing a smartphone produces about 55 kgCO2e and a laptop 260 to 300 kgCO2e. A share of those manufacturing emissions is attributable to each visit. Across thousands of visitors, this is the largest emission source, and free tools count it poorly, if at all.
2Annual emissions by traffic level, from a brochure site to a media site
As a first benchmark, the median home page weighs about 2.6 MB on mobile (HTTP Archive, 2025). Images account for about 35% of that, JavaScript for a quarter, and HTML, CSS, fonts and other resources for the rest. Page weight has almost tripled since 2015.
Annual footprint by traffic level
From 100 kg to 50 tonnes per year. Click to see the assumptions.
On the data centre and network alone, a page view emits 0.3 to 0.5 gCO2e. Over a year, a brochure site with 10,000 visits a month therefore emits 100 to 180 kgCO2e, an e-commerce site with 100,000 visits a month 1 to 2 tCO2e and a media site with 1 million visits a month 10 to 20 tCO2e. These figures cover only half of the impact: a full life cycle assessment (LCA), which includes user devices, multiplies them by 2 to 3.
To put these figures in context: 100 kgCO2e is the equivalent of a car journey from Paris to Bordeaux, and 1 tCO2e of a return flight from Paris to Marrakech. An SME's website accounts for far less than its business travel or its purchases. Optimising a site remains good practice, but if your Bilan Carbone® (the French carbon accounting method) shows 500 tCO2e and your site accounts for 0.2 of that, it is not the first project to tackle. On average, digital technology accounts for 2 to 5% of the footprint of a service-sector company.
3Website carbon calculators: reliable for comparing, not for absolute figures
Website Carbon Calculator is the best known: you enter the URL of a page and it estimates the CO2 emitted per page view. A typical result is about 0.36 gCO2e for an average page. Since July 2025 it has used version 4 of the Sustainable Web Design Model, which counts a share for visitors' devices, covering both use and manufacture, in addition to the data centre and the network. But everything is calculated from the volume of data transferred alone: the model takes no account of the type of device, of the work the page imposes on the processor or of visitors' actual behaviour (number of pages viewed, time spent). It works very well for comparing 2 versions of your site, and much less well for establishing your real impact.
EcoIndex is the main French reference tool, developed by the GreenIT collective. It gives a grade from A to G (like the energy label on a fridge) based on 3 factors: the weight of the page, the number of requests to the server and the complexity of the page being displayed. It therefore captures part of the load on the device, not just the network transfer. Like Website Carbon, it is reliable for comparing a page before and after optimisation, but not in absolute terms.
GreenIT Analysis (a browser extension from the same team as EcoIndex) goes further, with a page-by-page diagnosis and concrete recommendations. Lighthouse / PageSpeed Insights (Google) does not measure CO2 directly, but a fast site is generally less energy-intensive, so the two goals often coincide, though not always.
For the same page, results vary by a factor of 4 to 10 from one tool to another, because the models, scopes and assumptions about the electricity mix differ. So choose one tool and stick with it, tracking how the results change over time: comparing a Website Carbon figure with an EcoIndex figure is meaningless.
4Four misconceptions about website emissions, from email to green hosting
"One email = 4 g of CO2." That figure dates from 2010. An email without an attachment now emits 0.03 to 0.3 gCO2e. Spending 30 minutes sorting an inbox, by contrast, uses about 28 gCO2e in PC energy alone, more than deleting 1,000 emails could ever save: sorting an inbox is housekeeping, with no measurable effect on the climate.
4 misconceptions to correct
Click on a myth to see what the figures say.
"Our site is carbon neutral" (green badge). These badges rely on low-cost carbon credits (€3 to €5 per tonne) and bring no real reduction in emissions. In France, carbon neutrality claims (in French) have been regulated since 2023, and since 27 September 2026 the EU Empowering Consumers for the Green Transition (EmpCo) Directive has prohibited claims that a product or service has a neutral or reduced impact on the basis of offsetting. The Green Claims Directive, which was meant to go further, has been suspended since June 2025.
"Light page = green site." This shortcut is misleading: a 2 MB page with good caching has a cost on the first visit and almost none on later ones, because the browser keeps the files in memory. Conversely, a 100 KB page whose JavaScript keeps the processor running continuously uses more energy on the device. Page weight is an indication; what matters is the work the page imposes on the visitor's device.
"Green hosting solves the problem." A host that advertises "100% renewable energy" often buys certificates (guarantees of origin), which do not guarantee that the electricity consumed is renewable in practice. The criteria that count are the country where the data centre is located (low-carbon electricity or not), its PUE (ideally below 1.3) and its transparency on water consumption.
5Sustainable web design levers, in order of impact
Optimising images comes first: it is the simplest lever and can remove 25 to 40% of total weight. Images account for about 35% of the weight of a median page, often far more on a brochure site rich in photos. It takes only 3 actions: switch to the WebP or AVIF format (30 to 50% lighter than JPEG, with no visible loss), serve images at the right size for the screen (there is no point sending a 4K photo to a smartphone) and enable lazy loading (images load only when the user scrolls to them).
The 7 levers, ranked by gain-to-effort ratio
From the most cost-effective (top) to the most structural (bottom). Click to see "how to do it".
Reducing JavaScript saves 10 to 20%. JavaScript adds weight to the download and makes the visitor's processor work. Removing unused code and loading only what the page needs are the basics; the clearest gain often comes from dropping a complex framework on a 5-page brochure site, which almost never needs one.
Caching and a CDN (content delivery network) cut repeat transfers by 30 to 60%. With a well-configured cache, the second visit downloads almost nothing, since the browser reuses what it already has. A CDN brings content geographically closer to the user, which means less network distance, less energy and a faster site.
The choice of hosting provider has a large effect on the data centre emission source: moving to an efficient provider in a country with low-carbon electricity (Scaleway or OVH in France, Infomaniak in Switzerland) can cut these emissions by a factor of 5 to 10, thanks to the electricity mix and lower PUEs; it is a one-off change whose effects last for years.
6The French regulatory framework in 2026: RGESN and the REEN law
The RGESN (Référentiel général d'écoconception des services numériques, France's general framework for the eco-design of digital services) is the official French framework: 78 criteria for designing a website or an application responsibly. It was published in May 2024 by Arcep (the French electronic communications regulator) and Arcom (the French audiovisual and digital communication regulator). It remains voluntary, including for public services, but is increasingly used as a reference in public tenders and in the CSR (corporate social responsibility) policies of large companies. An RGESN audit takes 2 to 5 days.
The REEN law, France's law on reducing the environmental footprint of digital technology (réduction de l'empreinte environnementale du numérique), dates from November 2021. Since 1 January 2025 it has required local authorities with more than 50,000 inhabitants to adopt a digital sustainability strategy, and it regulates the energy efficiency and water consumption of data centres. It creates no reporting obligation specific to digital technology for companies: for them, digital technology falls within the existing climate reporting, namely France's mandatory greenhouse gas emissions report (BEGES) and, for companies that remain subject to it, the Corporate Sustainability Reporting Directive (CSRD).
In a Bilan Carbone®, the footprint of your site falls under Scope 3 (in French): under "purchased services" if a provider hosts your site, and under "use of sold products" if the site is your product, such as software as a service (SaaS) or a marketplace. If digital technology is a significant part of your business and you are subject to the CSRD, it will appear in that reporting as well. An RGESN audit then gives you material you can use directly.
7When to move to a life cycle assessment of a digital service
Free tools are ideal for getting started and tracking your progress, but they remain approximations: the margin of error on the absolute value can reach ±200%. A life cycle assessment (in French) is the complete scientific method: it includes the manufacture of equipment, the real electricity mix of your hosting provider and the measured behaviour of your visitors, and reduces the uncertainty to ±20-30%.
An LCA is justified when digital technology is your product (SaaS, marketplace, mobile app) and you need to document its impact for investors, in a public tender or in your CSRD reporting. For a targeted scope, the budget is €5,000 to €15,000 excl. VAT. If your website is one communication tool among others, free tools are more than enough to steer your eco-design.
8Where to start: test your pages, optimise your images, check your host
You can start now with 3 quick, free actions.
1. Run your site through EcoIndex or Website Carbon. Test your home page and your 3 or 4 most visited pages (check your analytics tool) and record the scores as a baseline. Repeat the test after 3 months with the same tool to measure progress.
2. Optimise your images. Convert them to WebP (most content management systems do this automatically in 2026), enable lazy loading and serve responsive sizes. On a WordPress site, a plugin such as ShortPixel or Imagify takes care of it. This can remove 25 to 40% of the page weight and makes the site load faster, which also helps your search rankings.
3. Check your hosting provider. Ask it for the PUE of its data centre and its electricity mix. If it cannot answer, or if the PUE is above 1.5, consider moving to a transparent hosting provider based in a country with low-carbon electricity (Scaleway or OVH in France, Infomaniak in Switzerland). If digital technology is central to your business, an RGESN audit or an LCA of your service will lay the foundations for a sound approach to your BEGES or CSRD reporting.
Main sources: HTTP Archive Web Almanac 2025 (page weight data), The Shift Project (a French think tank; 1-Byte model, sector data), ADEME Base Empreinte (ADEME is the French Agency for Ecological Transition; emission factors for digital equipment), NegaOctet (via ADEME Base Empreinte) (digital LCA methodology), RGESN (eco-design framework), Green Web Foundation CO2.js (calculation methodologies). The Légifrance and RGESN pages are in French.
