How it works
How we estimate a page's carbon
We measure how much data your page transfers, check whether your host is listed as green, and apply an open, published model. Here is each step, and what the result can and can't tell you.
Step 1
What we measure
Google PageSpeed Insights loads the public page, once as a mobile visitor and once as a desktop visitor, as a first visit with nothing cached. It records the bytes transferred during that initial load: the HTML, images, fonts, scripts and styles, including files from other companies' servers. We use whichever result is heavier, and the report shows both.
The test doesn't scroll, click or accept cookie banners. Images set to load lazily further down the page, content that appears after a click and tags that wait for cookie consent are usually not counted.
If the page can't be measured, we tell you and save nothing. We never fill in a guessed page weight.
Step 2
The green hosting check
We ask The Green Web Foundation whether your host is listed as green. If it is, the model removes the operational energy of the data centre from the estimate, which is about 18% of the total. If it isn't listed, or we can't confirm it, we say so plainly.
Step 3
How the estimate is calculated
We use the Sustainable Web Design Model v4. It estimates that transferring one gigabyte uses 0.300 kWh across three parts of the system. Each figure is operational energy plus embodied energy (the share of making the equipment), in kWh per GB:
- Data centres
- 0.055 + 0.012
- Networks
- 0.059 + 0.013
- Visitors' devices
- 0.080 + 0.081
- Total
- 0.300
We multiply that energy by a global average grid intensity of 494 g CO₂e per kWh, for one first-time visit. Gigabytes are decimal, so 1 GB is 1,000,000,000 bytes.
A worked example
A page that transfers 1 MB, on a host that isn't listed as green:
0.001 GB × 0.300 kWh/GB × 494 g/kWh = 0.148 g CO₂e
That is grade C.
Step 4
What the grade means
Grades follow the published digital carbon ratings for the Sustainable Web Design Model. Lighter pages earn better grades. The 1 MB example above lands in C, the middle of the scale.
Your report
What the report gives you
Emissions per page view
Our estimate of the CO₂e from one first-time visit to the page.
A carbon grade
Where that estimate sits on the A+ to F scale.
Green hosting status
Whether The Green Web Foundation lists the host as green.
Up to three improvements
Each one is backed by a measured Lighthouse finding, such as unused scripts or oversized images.
How we estimate this
A short disclosure in the report showing the figures behind the result.
You also get a one-sentence summary. Each rescan keeps the previous result in your history, and you can download the report as a PDF.
Limits
What the result can't tell you
It's an estimate, not a meter reading
We model energy from data transferred. We don't measure the electricity your servers or visitors actually use.
One first visit
We assume a first-time, uncached visit. Returning visitors often download less, so they usually produce less.
A global average grid
We use 494 g CO₂e per kWh for every page. Real grids are cleaner or dirtier depending on where and when the page is used.
A single lab test
PageSpeed Insights loads the page once from Google's servers. Results can vary between runs, and pages behind a login aren't covered.
Green hosting is a check of the host
It tells you whether the host is listed as green. It doesn't cover your visitors' devices or the networks in between.
Methodology version swdm-v4/gt-2026.10.