Can the CO₂ of a web page be measured precisely?

No: without direct energy data this is an estimate. The tool starts from the bytes transferred and applies the Sustainable Web Design Model v4, showing the model, its assumptions and its sources.

Which figures are measured and which are estimated?

Resource weight and Lighthouse metrics are lab data; CrUX, when available, contains aggregated data from real users; CO₂e is a modelled estimate. The annual scenarios are declared projections and can be replaced with your real page-load figure.

What does green hosting mean?

It means the domain appears in the Green Web Foundation's Green Web Dataset. That is a documentary check on the provider, not a real-time measurement of the energy used by each individual request.

Green Web Audit

How heavy is
your site, really?

Verifiable technical data, a CO₂e estimate declared as such, and a roadmap ordered by impact. No invented traffic, no numbers without a source.

SWDM v4 PageSpeed CrUX Green Web Foundation Web Risk

Result

The page at a glance

Estimated CO₂e / page load

–g

–

Estimate based on the bytes transferred.

Data transfer – –

Verified hosting – –

Core Web Vitals – –

Lab performance – –

Do this first

Actions ordered by estimated technical benefit, not by how easy they are to sell.

Where the bytes are

Total transferred –

Potential detected by Lighthouse

– avoidable bytes identified

Experience and performance

Lab and real users are kept separate: they answer different questions.

Lab

Synthetic test

–

A single controlled Lighthouse run. Useful for diagnosis, not for describing every user.

Field

Real users

–

CrUX aggregates real-world experience at the 75th percentile, when the public sample is large enough.

If the site grows

Page weight matters even with little traffic: if page loads go up, the impact grows in the same proportion.

CO2.js 0.19.0 SWDM v4 local calculation

Choose a page-load scenario

These are not visitors: one person can open several pages. Start from the closest case, then refine it if you know the real figure.

At 1,000 page loads/month –

If traffic grows ×10 –

Annual saving with 30% fewer bytes –

The projection will appear after the page has been analysed.

Sources, assumptions and limits

Every number has a type: measured, observed or modelled.

Want to go from report to code?

We can turn the priorities into a plan of work.

Let's talk

How to read the audit

A useful estimate
is still an estimate.

Digital emissions do not come from a universal meter. This audit applies the Sustainable Web Design Model v4 to the bytes measured by Lighthouse or, when that test is unavailable, to a limited estimate of the resources declared on the page.

Measured

Weight and Lighthouse metrics from the single run; in fallback, the weight is explicitly labelled as a direct estimate.

Observed

CrUX Core Web Vitals at p75, only when Google has a large enough public sample.

Modelled

CO₂e attributed to data centres, network and devices according to SWDM v4.

Why do we use traffic scenarios?

Free public sources give pageviews that are often incomplete or built on fragile heuristics. The buttons therefore show declared projections — not traffic attributed to your site — and you can replace them with your real page-load figure.

Does green hosting mean zero emissions?

No. The Green Web Foundation verifies the provider's energy declarations. SWDM only reduces the operational data-centre component; network, devices and embodied emissions stay in the model.

Is a single test enough to decide?

It is enough to find bottlenecks. For important decisions it is worth repeating the test, analysing several templates and comparing the results against RUM or Search Console data.

What does the URL reputation check cover?

Google Web Risk checks whether the URL appears in known threat lists, such as malware or social engineering. No match does not prove the site is free of vulnerabilities or compromises.