This translation has not been editorially reviewed yet. The German version is authoritative. Deutsche Fassung

CO₂ & EmpCo

How much CO₂ does a page view cause, and when is a page considered heavy?

Every view of a website costs electricity in the data centre, in the lines and on the visitor’s device. The check measures how many bytes a view transfers and estimates the CO₂ value per view from that. If a page transfers more than 2.5 MB, it reports that the page is heavy; in the report this finding is at the highest level of the CO₂ & EmpCo audit area.

How the check calculates

The basis is the Sustainable Web Design Model with the factors of its version 3: 0.81 kWh of energy per transferred gigabyte and 442 g CO₂ per kilowatt hour in the global electricity mix. If the host is listed at the Green Web Foundation with green electricity proof, the check, like version 3, calculates the data centres’ share of the energy demand (15 %) with 50 g per kilowatt hour and the remaining shares for network, devices and production with the global electricity mix, about 383 g per kilowatt hour in total. A 2.5 MB page comes to about 0.87 g CO₂ per view with the global electricity mix, about 87 kg at 100,000 views. A 1 MB page is at about 0.35 g per view. The report shows the page weight per view; the CO₂ value follows from it with this calculation.

What enters the score

The CO₂ score is made up of these measurements: page weight counts 45 %, the number of requests 25 %, the share of cacheable files 15 % and the JavaScript share 15 %. For weight, the full score applies up to 2.5 MB; above that it falls evenly to zero at 6 MB. A host with green electricity proof adds 8 points. If the database lists the host with grey electricity, that costs 8 points; for hosts outside the database the score stays the same. Every image from 200 pixels wide that is delivered more than 10 % wider than the display size and pixel density of the device require costs another point, at most five.

Where the 2.5 MB limit comes from

The HTTP Archive regularly measures millions of websites and publishes the median page weight. In October 2024 it was 2,311 KB on phones and 2,652 KB on desktop (Web Almanac 2024). The 2.5 MB limit lies between these values. For visitors on a phone, every additional megabyte means a longer loading time and used-up data volume.

Examples

Hero image

Not met

<img src="/bilder/team-original.jpg" style="width:1200px">
<!-- 6000 px breit, 4.800 KB -->

Passes

<img src="/bilder/team-1200.avif"
     srcset="/bilder/team-600.avif 600w, /bilder/team-1200.avif 1200w"
     sizes="(max-width: 1200px) 100vw, 1200px"
     width="1200" height="800" style="max-width:100%;height:auto"
     alt="Unser Team in der Werkstatt">
<!-- 140 KB -->

In the report, open the CO₂ & EmpCo audit area. It shows page weight, requests, JavaScript and image share, cacheability and hosting; the third-party server share has its own finding as soon as it is above 50 %.
Start with the images: deliver them at the displayed size, as WebP or AVIF, with loading="lazy" below the first screen.
Load videos only on a click and remove unnecessary scripts and fonts.
Switch on compression (Brotli or gzip) on the server for the document, scripts and style sheets.
Then order a re-measurement. The report then shows the new page weight.

Deutsche Fassung dieser Seite

FAQ

Is the CO₂ value an exact measurement?

The bytes are measured; the CO₂ value is an estimate. The model calculates with average values for data centres, networks and devices. It is suited to comparing pages with each other and showing progress between two measurements.

Does content from third-party servers count?

Yes, everything the browser loads during the view counts, including fonts, maps, videos and scripts from other providers.

Does the finding also apply to subpages?

Yes, every measured page is weighed. If one is above 2.5 MB, the finding is listed for that page.

Is page weight related to loading time?

Yes, a heavy page takes longer on a phone until the content appears. The loading time is in the Performance audit area; the causes are often found here.

Why does another CO₂ calculator show a lower value?

Many calculators use version 4 of the Sustainable Web Design Model with 0.30 kWh per gigabyte and 494 g CO₂ per kilowatt hour. This check calculates with version 3, that is, with 0.81 kWh and 442 g. By the version 4 method, the same page comes to about 40 % of this check’s value. You can still compare two measurements of the same page reliably, because both use the same version.