How is the carbon figure calculated?

Real transfer size is measured, then converted using Sustainable Web Design v3: 0.81 kWh per GB at 442 g CO2 per kWh, assuming 75% first visits and 25% returning visitors loading 2% of the page.

Is page weight measured or estimated?

Measured. The HTML is downloaded and each asset gets a HEAD request for its Content-Length. Assets that do not report a size are counted separately, so the total is a floor rather than a guess.

What is a good score?

The median web page is about 2.2 MB. Under 1 MB is cleaner than roughly 85% of the web. Images are almost always the biggest lever.

Website Carbon Footprint Checker

Website Carbon Footprint CheckerEvery page view moves data across networks, and moving data uses electricity. This tool measures what a page actually weighs and converts that into grams of CO2 per visit. It is free, needs no signup, and works on any public website.How the measurement worksMost carbon calculators estimate page weight. This one measures it. The HTML document is downloaded and its exact byte length recorded, then every script, stylesheet and image the page references is issued a HEAD request to read its Content-Length. Those figures are summed into a real transfer size.Assets that refuse HEAD requests, or that omit the Content-Length header, are counted as unmeasured and reported separately. This matters: it means the total is always a floor. The real page is at least this heavy and possibly heavier. Filling those gaps with an average would produce a rounder number and a less honest one.Up to forty assets are sampled. A page with three hundred assets does not need three hundred requests to be characterised, and issuing them would put needless load on someone else's server.The conversion modelTransfer size becomes carbon using the Sustainable Web Design model, version 3: 0.81 kilowatt-hours per gigabyte transferred, at 442 grams of CO2 per kilowatt-hour of global average grid intensity. Returning visitors are modelled as loading 2% of the page from cache, with a 75/25 split between new and returning traffic.Those assumptions are stated because a carbon figure without its assumptions is decoration. Change the grid intensity and every number moves.What this does not measureGreen hosting is not checked. A site on renewable power scores identically to one on coal with the same page weight, because the model applies a global average to everyone. Server-side compute, database queries and video streamed after page load are also outside the measurement. Read the result as a page-weight proxy, not a full lifecycle assessment.How to read the gradeThe median web page is roughly 2.2 MB, so that is the C boundary. Under 1 MB earns an A and puts a page cleaner than around 85% of the web. Over 8 MB is among the heaviest pages online.What actually reduces the number

Images — nearly always the largest share. Modern formats and correct sizing typically cut total weight by half.

Unused JavaScript — frameworks shipped whole when a fraction is used. Code splitting addresses this directly.

Web fonts — each weight and style is a separate download. Two weights is usually enough.

Third-party tags — analytics, chat widgets and ad scripts often outweigh the site's own code.

Caching — a good cache policy means returning visitors download almost nothing.

Related free tools

Tech Stack Detector — see which third-party scripts are adding the weight

HTTP Headers Checker — inspect cache and compression headers

Website Trust Score — security posture in one rating

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