We Audited 44 Websites for Digital Carbon Emissions. Here's What We Found.

Over the past month, we ran full PEER Audits on 44 websites — scanning 6,168 pages across businesses of all sizes, from global e-commerce brands to small professional services firms. We measured everything: carbon emissions per visit, page weight, Core Web Vitals, third-party bloat, green hosting, caching, image optimisation, and more.

The results tell a clear story: most websites are technically fast but environmentally wasteful. And the gap between the best and worst performers is staggering.

Here's what the data says.

The headline numbers

• Average CO2 per visit: 1.24 grams
• Average page weight: 9.7 MB
• Average wasted carbon: 35.5% of total emissions are preventable
• The efficiency gap: The cleanest site emits 0.05g per visit. The dirtiest emits 8.08g. That's a 149x difference.

To put that in context: if the worst-performing site in our sample receives 100,000 visits a month, it produces roughly 808 kg of CO2 annually — equivalent to driving a car from London to Edinburgh and back, four times. The cleanest site doing the same traffic? About 5.4 kg. Less than a return flight's worth of luggage surcharge.

Performance scores are high. Emissions scores are not.

This was the most striking finding. Across our 44 audits, the average scores per PEER pillar were:

Nearly every site we tested is technically competent. 97.7% pass Largest Contentful Paint. 95.5% have good server response times. These are not slow websites.

But speed and sustainability are not the same thing. A site can load fast while still shipping megabytes of unused JavaScript, unoptimised images, and dozens of third-party tracking scripts. The page arrives quickly — it's just far heavier than it needs to be.

Only 13.6% of audited sites scored an A grade on emissions. 45.5% scored D or worse.

Where the weight comes from

On average, the 9.7 MB loaded per page visit breaks down like this:

Images and JavaScript alone account for 86% of total bandwidth. These are the two biggest levers for any business serious about reducing their digital footprint.

The heaviest site we tested loaded 106 MB on its homepage — 745 times heavier than the lightest at 143 KB. Both are real businesses, both function perfectly well. One just does it with dramatically less waste.

Third-party scripts: the silent polluter

Every single website we audited — all 44 — loads third-party scripts. On average:

• 36 third-party requests per page
• 350 milliseconds of added latency per visit
• 4 MB of external resources per site

That's analytics platforms, tag managers, chat widgets, social embeds, advertising pixels, consent management tools, A/B testing frameworks, and more. Many of these fire on every page load whether or not anyone interacts with them.

Across our sample, we counted 1,619 third-party script requests totalling 178 MB of transferred data. The question every business should ask: how many of these are actually necessary?

The good news: green hosting is above average

65.9% of audited sites are hosted on green energy infrastructure, verified by the Green Web Foundation. That's roughly double the industry average of around 30%.

This likely reflects our sample — these are UK businesses, many with active sustainability commitments. But it still means a third of sites are running on traditional grid power, where every page view contributes to fossil fuel consumption.

Switching to a green host is one of the simplest, most impactful changes a business can make. It doesn't require redesigning anything. It's a hosting decision.

Render-blocking resources are nearly universal

43 out of 44 sites have render-blocking CSS or JavaScript — resources that prevent the page from displaying until they've fully loaded. We found 384 render-blocking resources across the sample, averaging 8 per site.

These slow down the user experience and increase energy consumption on the client device. The browser sits idle, waiting for files it may not even need for the initial view.

Layout instability is a quiet problem

81.8% of sites pass the Cumulative Layout Shift threshold (good), but the worst offenders are genuinely disorienting. Three sites scored above 0.25 CLS — meaning visible elements jump around the page as it loads. This isn't just a UX annoyance; every layout recalculation burns CPU cycles and battery.

What the cleanest sites do differently

The lowest-emitting sites in our sample share common traits:

1. Minimal third-party scripts — only what's genuinely needed
2. Properly sized and compressed images — WebP or AVIF, lazy-loaded below the fold
3. Code-split JavaScript — loading only what the current page requires
4. Green hosting — renewable energy infrastructure
5. Efficient caching — returning visitors don't re-download unchanged assets
6. Brotli compression — 77.3% of our sample use it, but the cleanest sites compress aggressively

Our own site, oynk.co.uk, emits 0.13g per visit. That's not because we have less content — it's because we've been deliberate about what gets loaded and how.

35% of your website's carbon footprint is probably unnecessary

This is the number that should concern every business leader reading this. On average, more than a third of the carbon produced by a website visit is wasted — resources loaded but never used, scripts executed but never needed, images downloaded but never seen.

That's not a technical limitation. It's accumulated neglect. Features added and never removed. Analytics tools stacked on top of each other. Images uploaded at 4000px wide and served to a 375px mobile screen.

The fix isn't a rebuild. It's a review. Most of this waste can be eliminated in days, not months.

What we'd recommend to any business

Based on 44 audits and 6,168 scanned pages, here are the five highest-impact actions:

1. Audit your third-party scripts. Remove anything that isn't actively providing value. If you can't explain what a script does, you probably don't need it.
2. Optimise your images. Serve modern formats (WebP/AVIF), compress properly, lazy-load below the fold, and never serve a 2000px image to a mobile device.
3. Switch to a green host. It takes an afternoon and eliminates the largest share of your operational carbon.
4. Review your JavaScript bundle. Code-split by route. Tree-shake unused dependencies. Don't ship a framework's kitchen sink when you need a teaspoon.
5. Run a PEER Audit. Know your numbers. You can't improve what you haven't measured.

The methodology

All audits were conducted using the PEER Audit framework developed by OYNK. Each site was scanned using headless Chrome with full network inspection, measuring real resource transfers, Core Web Vitals (lab-based), green hosting verification via the Green Web Foundation, and carbon calculations using the EcoPigs Digital Carbon Methodology v1.0 with real-time UK grid intensity data from the National Grid ESO API.

These are lab-based measurements. Real-world performance varies by device, connection, and location. But the relative comparisons hold: a site emitting 8g per visit in the lab isn't doing better in the wild.

If you'd like to know where your website stands, use our free Website Carbon Calculator for a quick check, or get in touch for a full PEER Audit.