If your web person suggests putting a content delivery network (CDN) in front of your site, it's natural to ask whether that makes the site greener as well as faster. The honest answer is: it depends on what the CDN is set up to do. Serving files from closer to your visitors makes pages quicker, but a carbon estimate based on the Sustainable Web Design Model counts the data transferred, not how far it travels. This article explains where a CDN can help your estimate, where it can't, and one side effect on the green hosting result that catches people out.
What a CDN actually does
MDN describes a CDN as “a group of servers spread out over many locations” that keep copies of your files so each request can be answered by a server near the visitor. Google's web.dev guide to CDNs lists what that brings:
- Proximity. CDN servers are closer to users than your own server, so each round trip is shorter and pages respond sooner.
- Caching. Files the CDN already holds don't have to be fetched from your server, which is faster and takes load off your host.
- Optional optimisation. Many CDNs can compress text files, and some can strip image metadata, compress images and convert them to newer formats such as WebP, or minify code.
The first two are about speed and server load. Only the third changes how many bytes reach the visitor.
Bytes, not miles
GreenTracer uses the Sustainable Web Design Model v4, which estimates energy per gigabyte transferred. The model has no input for distance: network energy is allocated by the amount of data, not by how far it goes. So a 2 MB page served from a CDN node down the road and the same 2 MB page served from a server on another continent get the same estimate, about 0.30 g CO₂e per view (2 MB at about 0.148 g per MB).
Caching at the CDN doesn't change that either. A CDN's cache sits on its servers, not in the visitor's browser, so a first-time visitor still downloads every file. GreenTracer measures exactly that: one initial, uncached load with Google PageSpeed Insights, on mobile and desktop, using the heavier run. A faster page and a lighter page are not the same thing, though they often go together.
That doesn't make speed unimportant. Google's guidance on improving Largest Contentful Paint says the best way to reduce the distance a resource travels is to use a CDN. Your visitors will notice. It just isn't a carbon saving in this model.
The settings that do reduce bytes
If you have a CDN, or are about to, ask your web person which of these are switched on. Each one can make the files themselves smaller, which is what the estimate counts:
- Text compression. web.dev recommends compressing text responses and prefers Brotli over gzip because it typically compresses further. Your server may already do this; the CDN can do it if not.
- Image optimisation. Automatic conversion to WebP or similar and metadata removal can shrink large photos. It isn't a substitute for uploading images at a sensible size in the first place: our guide to image weight covers that.
- Minification. web.dev suggests doing this when you build the site, with the CDN as a fallback.
To see whether it's working, open Chrome DevTools' Network panel with “Disable cache” ticked and compare the transferred total before and after, or run a GreenTracer check before and after the change. If the number doesn't move, the CDN is speeding things up without making them lighter.
How a CDN affects the green hosting check
GreenTracer's hosting result comes from the Green Web Foundation. Its check follows your domain's DNS records to an IP address and matches that address to a provider. With a CDN in front, the address belongs to the CDN, not your host.
The foundation explains the result in its guidance on CDNs and carbon.txt: the check can only see the address the CDN returns. If the CDN isn't a verified provider, a site on a green host can come back as “no evidence found”. If the CDN is verified, the result names the CDN rather than your host. Either way, the result describes the CDN in front of the site rather than the server behind it.
The foundation's answer is carbon.txt, which lets providers link domains to their own evidence instead of relying on IP addresses. If your host is green but your result says green hosting “could not be confirmed”, ask your host and CDN whether they support it. Our article on what green hosting changes in your estimate explains why the hosting result moves the number by about 18%, not more.
Should you use one?
Use a CDN if your site is slow for visitors far from your server, or if your host struggles under traffic. Those are good reasons on their own. Don't expect it to change your grade unless it also sends fewer bytes, and check what the green hosting check says afterwards. If you want a lower estimate, the bigger lever is still the page itself: fewer and smaller images, fonts and scripts. Every megabyte removed lowers the estimate by about 0.148 g CO₂e per view, or about 0.121 g on a host listed as green, whoever delivers it.
Check your page before and after any change, so you can see what actually moved.
