Somebody will eventually ask you if you have ever run a website carbon audit, and what a website carbon audit measures is the question that follows straight after.
A procurement questionnaire, a tender, a client with reporting obligations of their own. The wrong answer is a number you got from a free tool three months ago and cannot explain. The right answer is a figure, a method, a date and a set of conditions.
A website carbon audit is not complicated. It is mostly the discipline of writing down what you did, which is the part that turns a number into evidence rather than a guess dressed up in decimals.
What a website carbon audit measures
Not emissions. What a website carbon audit measures is an estimate derived from them.
Carbon calculators take the data your page transfers and apply modelled assumptions: the energy required to move and process that data, the carbon intensity of the electricity involved, and whether the hosting is renewable-powered. None of that is measured directly at the point of use. All of it is modelled from measured inputs, chiefly page weight.
Published models such as the Sustainable Web Design methodology give figures for the emissions associated with an average page view, and those figures have been revised as the underlying research has developed. Any number you quote should name its source and its version, because the same page assessed under two methodologies produces two different answers, and neither one is wrong. It is simply a different model.
The method, step by step
- Choose your pages. Your five to ten most-visited, from analytics rather than intuition. The homepage is rarely the whole story.
- Record the conditions. Which device, which connection, first or repeat visit, and the date. Without these, nothing you measure later is comparable.
- Measure page weight and requests. Browser network panel, cache disabled, sorted by size.
- Measure Core Web Vitals. Under the same conditions as the page weight check.
- Run a carbon estimate. One tool, applied consistently. Note which tool and which version of its underlying model.
- Check your hosting. Whether your provider claims renewable power, and what that claim actually covers. The Green Web Foundation’s hosting check tells you whether a host appears in its directory, which is a starting point rather than a full verification.
- Multiply by traffic. A heavy page with fifty views a month matters less than a moderate page with fifty thousand.
Write all of it down in one place. That document is your baseline, and the thing you will be glad of in six months when someone asks how you got the number.
If you want a starting figure before working through the full method yourself, our free website carbon audit gives you a first read on where your site stands, using this same method. It costs nothing and takes a few minutes to request, and it sits ahead of a full website carbon audit if you decide you need one.
What the results of a website carbon audit show
Three patterns turn up on almost every site that has never been reviewed.
Images dominate page weight, and most are served larger than they display, in dated formats, loaded eagerly whether or not the visitor ever scrolls to them.
Third-party scripts account for more than anyone expects, and several belong to tools nobody has opened in a year.
The homepage is rarely the worst page. Product pages, galleries and anything built by a marketing campaign usually are, because they accumulate images and embeds faster than anyone remembers to prune them.
Fixing what you find
In order of return: images, then unaccounted third-party scripts, then fonts, then JavaScript.
Re-measure after each change under the same conditions you recorded at the start, so you can attribute the improvement rather than guessing at it. A website carbon emissions audit that skips this step produces a before figure and an after figure that cannot honestly be compared.
Where performance and environmental impact both apply, treat them as the same lever. Reducing unnecessary data transfer and processing supports a faster experience while lowering the estimated figure, though the environmental side of that stays modelled rather than directly measured. Our website sustainability audits work through exactly this list, in this order, and document the before-and-after so the improvement is defensible rather than asserted. Checking your host’s own footprint is worth doing alongside this; our guide to green web hosting in the UK covers what a renewable claim should actually cover.
Why local benchmarks do not exist
You may want to know how your site compares to other businesses in your area. No credible dataset for that exists, and anyone offering one should be asked how they gathered it.
What does exist is national and global data on page weight across the web, which is a legitimate comparison. Our guides to website carbon footprint and the carbon footprint of websites set out what that wider data actually shows. Your own trend over time is more useful still, because it controls for everything specific to your sector and audience. If you want the fuller picture of what a website’s environmental impact covers beyond the page-weight figure, our piece on website environmental impact goes into that.
Reporting it safely
Environmental claims must be accurate and substantiated. The Competition and Markets Authority’s Green Claims Code sets out the standard, and it applies whether the claim sits on your own site or in a client-facing report.
State the figure, the tool, the conditions, the date and the scope. Say plainly that the figure is a modelled estimate covering website delivery rather than your organisation’s wider digital footprint. That framing is both accurate and more credible to anyone assessing it, because it demonstrates you understand what you measured rather than repeating a number you found somewhere.
If you would rather start from a result than build the method yourself, request our free website carbon audit and we will run the first pass for you. Our website sustainability audits follow this same method in full and document it, which is what makes the output usable in a tender.