It is a saving you can make yourself
You cannot change the grid. You can change what your site sends. Take a 3 MB page down to 1.5 MB and the estimate halves on every visit, with nothing to offset.
Website carbon footprint and page weight checker
Website carbon calculator How heavy is your page?
Every page view moves data through a data centre, a network and a device, and all of that uses electricity. Paste in any public page to see how much it transfers, roughly what that costs in CO₂e per view, and where the weight is.
Example result
Lighthouse run,
CO₂e per page view
gSustainable Web Design Model v4
Data transferred
Network requests
Completed on page load
Green hosting
Speed, from the same run
Lighthouse also scores performance, and Google publishes Core Web Vitals from real Chrome users. The same bytes drive both, so they sit beside the carbon result.
Lab score
- LCPLargest Contentful Paint
- INPInteraction to Next Paint
- CLSCumulative Layout Shift
More from this Lighthouse run
- First Contentful Paint
- Speed Index
- Total Blocking Time
- Server response time
- LCP, lab
- CLS, lab
Lab figures come from one simulated mobile or desktop load in Google's test environment and vary between runs. Core Web Vitals above use the 75th percentile of real visits over the last 28 days where Chrome has enough data. Google's page experience signals use the field data, not the lab score.
Where the weight comes from
Bytes transferred by resource type, as compressed on the wire. Start with the biggest category.
Monthly estimate
Per-view figures are small. Multiply by your traffic to see what they add up to.
CO₂e per month
Data transferred per month
Reduction simulator
In this model emissions move in step with bytes. Halve the page and the estimate halves.
CO₂e per view at target
CO₂e saved per month at the traffic above
Where to start
These come from fixed rules applied to the breakdown. They tell you which category to look at first, not exactly what to change.
One Lighthouse run through the PageSpeed Insights API, with no scrolling or interaction. Results are cached for six hours. The estimate uses CO2.js with the Sustainable Web Design Model v4 and the Digital Carbon Ratings scale. Green hosting comes from The Green Web Foundation and only changes the data-centre share of the model.
BackgroundData has weight
What is a website carbon footprint?
A web page has weight. Each visit pulls files from a server, through the network, onto a phone or laptop, and every step runs on electricity.
A website carbon footprint is an estimate of the emissions caused by loading a page, and it is driven almost entirely by how much data the page sends. Images, video, fonts and JavaScript are usually where the kilobytes go.
The figure is given in grams of CO₂ equivalent per page view. One view is tiny. A page with 100,000 views a month is loaded over a million times a year, so a saving of a few hundred kilobytes repeats a million times too.
Page weight is the part of this you actually control.
The estimate here uses the Sustainable Web Design Model, the open method Website Carbon and Screaming Frog also use, so you can compare it with other tools on the current version of the model.
- 2.6 MBMedian mobile home page, July 2025. Up 8.4% on the year before.HTTP Archive Web Almanac 2025
- 0.33 gMedian CO₂e per mobile page view, measured across millions of sites.Web Almanac 2024, Sustainability
- 1.5%Global electricity used by data centres in 2024. The IEA expects that to double by 2030.IEA, Energy and AI, 2025
- 10%Mobile sites on hosting verified as green. The rest are not.Web Almanac 2024, Sustainability
The scaleA+ to F
The seven grades, and where they come from
The letter is the Digital Carbon Rating published by Sustainable Web Design. Each threshold is a percentile of real pages in the HTTP Archive: an A+ page is lighter than 95% of the web, an F is heavier than half of it.
Why it mattersThree practical reasons
Why it is worth measuring
Estimates put the internet's electricity use around 1,000 TWh a year once you count data centres, networks and the devices people use. Pages are getting heavier every year, and each extra kilobyte is served over and over.
Someone will ask
Sustainability reports, ESG questionnaires and tender documents now include questions about digital operations. A measured figure, a plan to reduce it and green hosting are answers. A paragraph about caring is not.
Visitors feel it first
A lighter page loads sooner on a weak signal or an old phone, costs less in bandwidth and keeps people reading. The work that reduces emissions is the same work that makes a site pleasant to use.
SearchOne job, two results
Lighter pages and SEO
Google does not rank pages by carbon rating. It does look at page experience, and the metrics behind it depend on the same bytes this tool counts.
Core Web Vitals measure how quickly the main content appears, how fast the page responds to a tap, and whether things jump around while loading. Google calls these Largest Contentful Paint, Interaction to Next Paint and Cumulative Layout Shift, and says good scores here, with other page experience signals, are what its ranking systems try to reward. Oversized images slow the first. Heavy scripts slow the second. Fonts and embeds that arrive late cause the third.
Cutting page weight helps all three. It also makes the site cheaper to crawl: a bot that has to fetch 5 MB per page covers far less of a large catalogue than one fetching 500 KB.
Images at 60% of the transfer is your biggest emissions saving and your most likely Largest Contentful Paint problem, in one line.
- Resize and compress imagesServe AVIF or WebP at the size they are displayed. On most pages this is the biggest win.
- Remove JavaScript you do not useDrop dead bundles, defer anything not needed for the first paint, and question every third-party tag.
- Load below the fold laterSend what the visitor sees first. The rest can wait until they scroll.
- Go easy on fonts and videoTwo weights in WOFF2 are enough. Video should start on a click, not on page load.
- Cache properlyLong cache lifetimes and a CDN mean a returning visitor downloads almost nothing.
- Check your hostingVerified green hosting lowers the data-centre share of the estimate. It takes a minute to check.
HostingThe data-centre share
What green hosting means
Green hosting means the servers that store and send your pages run on renewable electricity, or on power the provider matches with renewable energy purchases.
The Green Web Foundation keeps the public register, which lists hundreds of providers. A provider is listed only after it submits evidence, such as renewable energy certificates, power purchase agreements or supplier statements, and that evidence is reviewed. The calculator checks the domain you enter against this register, so "Not verified" means the host has not been listed, not that it necessarily runs on fossil power.
In the model behind this tool, hosting only affects the data-centre share of the estimate, about a fifth of the total. It is still worth choosing, because it is a one-time decision that applies to every page view from then on.
Two things to watch. A site fronted by a green content delivery network can pass the check even when its origin server is not green, because the check sees the network edge. And some large clouds are green in the register while others are not, so the same platform choice can land either way.
A few examples from the register
Six providers whose own domains returned a green result when checked on 17 September 2026. This is a sample to show the range, from global clouds to regional hosts, not a ranking or a complete list.
- Google CloudCloud platform. Also covers sites hosted on it, such as SiteGround.cloud.google.com
- CloudflareNetwork and hosting. Many hosting brands pass the check through Cloudflare's edge.cloudflare.com
- HetznerGerman and Finnish data centres. Also behind Bunny.net.hetzner.com
- InfomaniakSwiss hosting and cloud.infomaniak.com
- KrystalUK hosting.krystal.io
- OVHcloudFrench cloud and hosting.ovhcloud.com
Check your own host with the Green Web Check, or browse the full register in the Green Web Directory.
QuestionsBefore and after a test
Questions people ask
What is a good website carbon score?
On the Digital Carbon Ratings scale, A+ is 0.040 g CO₂e per view or less and A goes up to 0.079 g. The median page on the web is roughly 0.33 to 0.37 g, which is a D. A page under about 1 MB of transfer will usually land at A or B.
Is website carbon a Google ranking factor?
No. Google has never named carbon or sustainability as a ranking signal. It does reward good page experience, and the quickest route to better Core Web Vitals is sending fewer bytes. Lower carbon comes with it.
How accurate is the estimate?
It is a model, not a meter. Measured data transfer is multiplied by an average energy figure per gigabyte and a global average grid factor. That makes it good for comparing pages and tracking change, and rough as an absolute number for any one site. The people who maintain the model say it tends to overestimate rather than underestimate.
Why is my result different from Website Carbon or Screaming Frog?
Usually one of three things: the page was measured at a different moment, in a different environment (this tool runs Lighthouse once, without scrolling), or with a different version of the model. Version 4 gives lower grams and stricter letter thresholds than version 3, so the same page can be a C in one tool and an E in another.
Does green hosting make my site carbon neutral?
No. Hosting only affects the data-centre share of the model, about a fifth of the total. The network and the visitor's device still draw power from wherever they are. Green hosting is worth having. Page weight is the bigger lever.
What does the calculator miss?
Anything that loads after scrolling or clicking, logged-in and personalised content, scripts that wait for cookie consent, and video that streams after the initial load. A real visit to a media-heavy page is often heavier than the figure shown.
MethodMeasured, modelled, rated
What the number means, and what it leaves out
The estimate is a proxy built on data transfer. Use it to compare pages and track change. It is not a measured emission.
Measured by Lighthouse
Google's PageSpeed Insights API loads the page in a controlled environment and reports every network request with its compressed size. Content that only appears on scroll, logged-in views and consent-dependent scripts are not included.
Converted with CO2.js
The Green Web Foundation's library applies the Sustainable Web Design Model v4: energy per gigabyte across data centres, networks and devices, including the energy embodied in the hardware, multiplied by a global average grid intensity.
Rated on a shared scale
The letter comes from the Digital Carbon Ratings published by Sustainable Web Design. A+ is 0.040 g per view or less; F is 0.360 g or more. Tools still on the v3 scale will show different letters for the same page.
Sources and where each number comes from
- Sustainable Web Design Model v4The open methodology behind the estimate. It sets the energy per gigabyte for data centres, networks and devices, the embodied share, and the grid intensity. Maintained by Wholegrain Digital and Mightybytes with the W3C Sustainable Web Interest Group.sustainablewebdesign.org
- Digital Carbon RatingsThe A+ to F scale and its thresholds. Each grade is a percentile of pages in the HTTP Archive, so the letters describe where a page sits against the rest of the web rather than an arbitrary target.sustainablewebdesign.org
- CO2.js, The Green Web FoundationThe open-source library that turns bytes into grams and applies the rating. This tool uses the current release with the v4 model as default. Screaming Frog and Website Carbon use the same library.developers.thegreenwebfoundation.org
- Green Web CheckThe Green Web Foundation's dataset of hosting providers verified as running on renewable energy. The tool queries it by domain; a "not found" result means the host has not been verified, not that it is necessarily fossil-powered.thegreenwebfoundation.org
- PageSpeed Insights APIGoogle's public interface to Lighthouse. It loads the page in a controlled lab environment and returns every network request with its compressed transfer size. That list is what gets summed.developers.google.com
- HTTP Archive Web AlmanacAnnual analysis of millions of real pages. The 2025 Page Weight chapter supplies the median page size and its growth; the 2024 Sustainability chapter supplies the median CO₂e per view and the share of sites on green hosting.almanac.httparchive.org
- IEA, Energy and AI (2025)The International Energy Agency's figures for data centre electricity use in 2024 and its projection to 2030, used for the 1.5% figure above.iea.org
- Google Search CentralGoogle's own guidance on Core Web Vitals and page experience, quoted in the SEO section. It is the basis for saying page experience is rewarded and carbon is not a ranking signal.developers.google.com
Next stepFrom a number to a lighter site
From estimate to plan
Every kilobyte you drop
is one you never send again
The number above is a starting point. The useful part is knowing which images, scripts and hosting choices on your site are worth changing, and in what order. That is the work I do, and it is the same work that makes a site faster and easier to find.
Discuss your siteSee how technical SEO works here