Digital sustainability is not a well-known idea. We’ve raised it in enough conversations now to be fairly sure of that, and not just with people outside the field. Some of the people least familiar with it work in sustainability.
That got us wondering about the other end of the scale. If the concept hasn’t reached the people whose job it is, has it reached the organisations with the largest sustainability teams, the most detailed reporting obligations and the most scrutinised environmental commitments in the country?
So… we measured the homepages of the FTSE 100 to find out.
A note before we start: this isn’t a criticism of anyone. We haven’t set out to catch companies saying one thing and doing another, and as it turns out, the data doesn’t support that story anyway. What we wanted to know was simpler. Is the digital layer part of the sustainability conversation at all, or does it sit outside it? Every company named below is named because the number is interesting, not because we think anyone has done anything wrong.
What we did
Every time someone loads a web page, energy is used somewhere. Data moves across networks, servers respond, devices render what arrives. One visit is trivial. Homepages are not one visit.
On 3 July 2026 we crawled the homepages of the FTSE 100 and recorded two things for each one.
First, page weight, meaning the total size of everything the page loads. We converted that into an estimated carbon figure per visit using standard transfer-based methods. Second, whether the words “sustainability” or “environment” appear anywhere on that page.
That second test is deliberately generous. A single word counts. We’re not assessing the quality of anyone’s environmental strategy, and a keyword match tells you nothing about whether a company is doing good work. It only tells you whether the topic is present where most people arrive.
Some caveats worth stating plainly, because they affect the numbers. Four companies blocked our crawler, six others timed out during the crawl. That’s routine bot protection rather than anything unusual, meaning we were only able to capture complete page weight for 90 of the 100 sites.
Nearly nine in ten mention it
87 of the 99 companies we checked, or 88%, mention sustainability or the environment on their homepage.
That figure is roughly what we expected. Sustainability messaging is now a standard part of how large organisations introduce themselves, and the homepage is where that introduction happens.
Most of those homepages are heavy
The average FTSE 100 homepage weighs 8 MB and produces around 2.25g of CO₂ per visit.
The average is the least useful number we have, though, because the spread is enormous. The median homepage is 4.15 MB, barely half the mean. Almost twice as many sites sit below the average as above it, and the ones above it are very far above it.
At one end sits Babcock International at 0.55 MB. At the other, Vodafone at 59 MB. That’s a 108-fold difference between two companies in the same index.
The Digital Carbon Rating System grades pages from A+ down to F. Anything from A+ to E is lighter than the average web page globally. An F simply means the page is heavier than that global average, so it’s a comparison rather than a verdict.
| Grade | Companies |
|---|---|
| A | 2 |
| B | 2 |
| C | 6 |
| D | 6 |
| E | 11 |
| F | 63 |
63 of the 90 homepages we measured are heavier than the average web page. Two score an A.
The weight is also highly concentrated. The ten heaviest homepages account for 40% of the total page weight across all 90 sites. The twenty heaviest account for 60%. If those ten came down to the index median, a third of the total weight would disappear.
The two findings are unrelated
This is the part we didn’t expect.
We assumed we’d find some relationship between the two measurements. Perhaps companies talking about sustainability would run slightly lighter sites, or the heaviest sites would turn out to be the quietest ones. Neither is true.
| Companies | Median homepage | |
|---|---|---|
| Mentions sustainability | 80 | 4.10 MB |
| Doesn’t mention it | 10 | 4.19 MB |
Those are the same number. Statistically there is no meaningful difference between the two groups (Mann-Whitney U, p = 0.68).
Some supporting figures point the same way:
- Of the 80 companies that mention sustainability, 55 are heavier than the global average and one scores an A
- Nine of them run homepages over 20 MB
- Of the 23 heaviest homepages in the index, 19 mention sustainability, almost exactly the rate across the whole sample
Whether a FTSE 100 company talks about sustainability on its homepage tells you nothing at all about how heavy that homepage is.
The same pattern in the sectors closest to the subject
If the connection existed anywhere, we’d expect it in the industries where environmental performance is central to the business. It isn’t there either.
Water and energy utilities
| Company | Homepage | Grade | Mentions sustainability |
|---|---|---|---|
| National Grid | 3.13 MB | F | Yes |
| United Utilities | 3.53 MB | F | No |
| Severn Trent | 4.63 MB | F | Yes |
| SSE | 4.92 MB | F | Yes |
| Centrica | 10.30 MB | F | Yes |
Every UK utility in the index is heavier than the global average. Four of the five mention sustainability on the homepage. The one that doesn’t, United Utilities, sits in the middle of the group on weight.
Mining and energy
| Company | Homepage | Grade | Mentions sustainability |
|---|---|---|---|
| BP | 2.26 MB | D | Yes |
| Glencore | 2.90 MB | E | Yes |
| Fresnillo | 4.16 MB | F | Yes |
| Anglo American | 8.54 MB | F | Yes |
| Antofagasta | 22.80 MB | F | Yes |
| Endeavour Mining | 23.92 MB | F | Yes |
All six mention sustainability. Their homepages range from 2.26 MB to 23.92 MB, a ten-fold spread within one sector. BP and Glencore are among the lighter sites in the whole index. Antofagasta and Endeavour Mining are both inside the ten heaviest.
Same sector, same scrutiny, same messaging, entirely different results. Which is the clearest illustration we found that page weight is being decided somewhere other than the sustainability conversation.
Why the two don’t connect
We think the answer is structural rather than deliberate.
Homepage weight is the product of decisions made over years by people who were never asked to think about carbon. Images get uploaded at higher resolution than the page needs. A JavaScript framework loads to support a feature most visitors never touch. Analytics, consent management, personalisation and advertising tags accumulate, each added for a good reason and rarely removed. Fonts and video arrive from third-party domains. None of this is unusual and none of it is careless. It’s what a large website looks like after a decade of normal work.
Meanwhile, sustainability reporting counts buildings, travel, fleet and supply chain. No major reporting framework asks a company what its website weighs. There’s no line item, no owner, and no internal pressure to look.
So the website sits in a gap. Marketing owns the content. IT owns the infrastructure. The sustainability team owns the reporting. The carbon cost of delivering the message doesn’t belong to any of them.
It’s a solvable problem
The most useful thing in this dataset is the evidence that heavy homepages aren’t inevitable.
| Company | Homepage | Grade |
|---|---|---|
| Babcock International | 0.55 MB | A |
| Pershing Square | 0.63 MB | A |
| Aviva | 0.93 MB | B |
| Melrose | 1.01 MB | B |
| AstraZeneca | 1.57 MB | C |
These are not small or simple organisations. A global pharmaceutical company and a FTSE 100 insurer both manage it comfortably. They have the same complexity, brand requirements and stakeholder demands as anyone else in the index. They just have lighter websites.
Getting there rarely means rebuilding. It usually means compressing images properly, removing scripts nobody uses any more, auditing third-party tags, and being deliberate about what genuinely needs to load before someone can read the page.
Those changes reduce emissions. They also make pages load faster, work better on poor mobile connections, cost users less data and perform better in search. That combination is unusual. Most sustainability work costs something in exchange for the benefit. This one mostly pays for itself.
What it adds up to
A single page load producing 2.25g of CO₂ isn’t, on its own, worth anyone’s attention.
Here’s a deliberately conservative illustration. Take the 90 homepages we measured, and assume each is viewed just ten times a day. Together they’d produce roughly 2 kg of CO₂ daily, and around 739 kg a year.
Ten views a day is absurdly low for companies of this size, and homepages are a tiny fraction of the sites they sit on. Each of these organisations runs hundreds or thousands of pages, all with their own weight and their own traffic. The real figure isn’t in the same range. That’s rather the point: even a floor set this low produces something worth measuring.
The open question
The measurement side of this is getting easier. Page weight has always been visible to anyone who wanted to look, and the tools for turning it into a carbon estimate are now quick to run. As that continues, the distance between a company’s stated environmental position and the efficiency of the site stating it becomes harder to leave unexamined.
For now, the picture is fairly clear. Sustainability is on almost every FTSE 100 homepage. Very few of those homepages are built with it in mind. And the two facts have nothing to do with each other.
Whether that changes depends on something quite mundane: whether anyone decides the website belongs on the list of things worth counting. Given the impact the internet has on global emissions (double that of aviation), we think it’s time that websites are added to that list…
Find out what your own homepage weighs
You can run the same test we did. Paste a URL into our Website Carbon Checker and you’ll have a page weight, an emissions estimate and a grade in about thirty seconds. It’s free and there’s nothing to sign up for.
We built it, so treat that as the declared interest it is. It runs on the Sustainable Web Design Model, which is open source and maintained and updated by a group of organisations, rather than a single one.
Either way, the number is only the starting point. The more useful question is what’s behind it, and that’s usually a shorter list than people expect. If you’d like someone to look at yours and tell you what we’d do about it, get in touch.
Methodology: We crawled the homepages of the FTSE 100 on 3 July 2026, using Screaming Frog, recording total page weight and checking each page for the terms “sustainability” and “environment”. Emissions are calculated using the Sustainable Web Design Model, the open-source methodology published at sustainablewebdesign.org. Grades follow the Digital Carbon Rating System, which runs from A+ to F, where F indicates a page above the global average. Ten companies blocked automated crawling, and complete page weight data was captured for 90.
