Measured, not estimated
What this website costs the planet
Every website has a carbon cost: moving bytes takes electricity, in the data centre, across the network and in your device. The honest way to talk about it is to measure this page, right now, in your browser — so that is what the figure below is.
How that figure is worked out
- Bytes. The browser’s Resource Timing API reports what this page actually transferred, including anything already in your cache (which counts as no new bytes, because it is).
- Grams. co2.js from the Green Web Foundation converts those bytes using the Sustainable Web Design model, version 4.
- Green hosting. The site runs on Hetzner, which the Green Web Foundation lists as a green host with renewable-energy and ISO 14001 evidence on file — you can check that yourself. Under this model that lowers the figure by roughly 18%, because only the data-centre part of the chain is affected — not the network or your device.
- What we can’t see. Browsers hide the size of some third-party responses. Where any of those exist on a page, the figure is labelled a minimum and the hosts are named, rather than quietly counted as zero.
Low-carbon mode
The leaf in the header serves you a deliberately lighter version of the site. It is set to Auto by default, which means it stays off unless there is a reason to turn it on.
- Lighter images. Photography is the heaviest thing on most pages here. In low-carbon mode the image optimiser re-encodes at a lower quality — measured at about a quarter of the usual weight on our home-page photograph.
- No motion. Entrance animations and transitions are switched off. That is not bytes, but redrawing the screen costs energy on your device.
- No decorative extras. Imagery that carries no information is dropped before it is ever requested.
On Auto it switches itself on if your device or browser asks for reduced data, or if the electricity grid is running dirty:
Checking the current UK grid carbon intensity…
Grid data from the National Grid ESO carbon-intensity API, cached for half an hour — the interval at which it updates.
Choices that keep every page light
Low-carbon mode is the opt-in version. These apply to everyone, all the time.
- Photographs are served as AVIF or WebP at a quality chosen to be visually indistinguishable rather than maximal, and cached for thirty days once encoded.
- Pages are rendered ahead of time and served from cache, so a visit rarely costs any database work at all.
- Fonts are self-hosted, and only the two faces that draw the largest text are preloaded.
- The interactive map loads only when you ask for it, and only after you have accepted the cookies it needs — so most visits never download it.
- Below-the-fold sections are skipped by the browser’s layout engine until they are near the viewport.
