How does it work?
Calculating a website's carbon footprint is far from simple. We aim to be completely transparent about how we do it: every constant, every source, every caveat. Treat the results as a well-founded estimate, not precise science.
How do you measure a website?
When you scan a domain, we load its homepage with Google Lighthouse (via the PageSpeed Insights API) on a simulated mobile device. Lighthouse reports exactly how many bytes were transferred to display the page, broken down by resource type: images, JavaScript, CSS, fonts and so on. That transfer size is the foundation of everything below.
We also query The Green Web Foundation's registry to find out whether the site's host runs on verified green energy.
How do you calculate CO2 emissions?
We use the Sustainable Web Design Model, version 4 (SWDM v4), an open, peer-reviewed methodology maintained by the sustainable web community and used by tools across the industry. It evolved from the same research our original calculator was built on (Andrae's New perspectives on internet electricity use), so our v1 and v2 results share DNA, but v2 uses newer, better data.
The model attributes energy use to three segments, each with an operational part (running the equipment) and an embodied part (manufacturing it):
| Segment | Operational | Embodied |
|---|---|---|
| Data centres | 0.055 kWh/GB | 0.012 kWh/GB |
| Networks | 0.059 kWh/GB | 0.013 kWh/GB |
| User devices | 0.080 kWh/GB | 0.081 kWh/GB |
Energy is converted to CO2e using the global average grid intensity of 494 g CO2e per kWh (Ember, global electricity data). If the site is verified green-hosted, the data centre's operational energy is treated as zero-carbon. Embodied emissions remain, because servers still have to be manufactured.
The formula
C= CO2e in grams per page view (first visit, the number we grade)Cv= CO2e in grams per visit, adjusted for returning visitorsb= The total number of bytes transferredGB= Gigabytes transferredI= Global average grid intensity (494 g CO2e/kWh)g= 0 if the host is verified green, otherwise 1Od,On,Ou= Operational energy of data centres, networks and user devices (0.055, 0.059, 0.080 kWh/GB)Md,Mn,Mu= Embodied energy of the same three segments (0.012, 0.013, 0.081 kWh/GB)F= Estimated share of first-time visitors (0.75)R= Estimated share of returning visitors (0.25)s= Share of data a returning visitor re-downloads (0.02)
GB = b109
C = GB · I · ((Od · g + Md) + (On + Mn) + (Ou + Mu))
Cv = (C · F) + (C · s · R)
A worked example
A 1 MB page on standard hosting: 0.001 GB × 0.300 kWh/GB × 494 g/kWh ≈ 0.148 g CO2e per view: a C rating, just over the B threshold.
Returning visitors
The headline number (and your grade) is for a first-time visit. The per-visit figure Cv assumes 25% of visits are returning visitors who re-download only ~2% of the data thanks to caching, the same assumption our v1 calculator used.
How do you grade websites?
Grades map to fixed thresholds from the SWDM v4 digital carbon ratings, derived from HTTP Archive percentiles of real-world pages. Your grade only depends on your own emissions; it can never be pushed down because other sites scanned better. (Our v1 grades were relative to other scanned sites; we changed this on purpose.)
| Rating | g CO2e per page view | Meaning |
|---|---|---|
| A+ | ≤ 0.040 | Cleaner than 95% of the web |
| A | ≤ 0.079 | Cleaner than 90% |
| B | ≤ 0.145 | Cleaner than 80% |
| C | ≤ 0.209 | Cleaner than 70% |
| D | ≤ 0.278 | Cleaner than 60% |
| E | ≤ 0.359 | Cleaner than 50% |
| F | > 0.359 | Heavier than half the web |
Which pages do you test?
We test the homepage of the domain you enter and treat the result as representative for the site. Any publicly reachable website can be scanned; results are cached for 7 days (you can re-scan from a site's report page).
I use green hosting, but the test says otherwise. Why?
- If your hosting provider hasn't registered your server's IP range with The Green Web Foundation, we can't detect it. Ask your provider to register.
- Behind a CDN, we see the CDN provider as the host, not your origin server.
- Occasionally the registry is temporarily unreachable. When that happens we flag it on your report and assume grid power. Re-scan later.
Note that the registry includes hosts that buy grid electricity but offset it; like The Green Web Foundation, we treat these as green.
The scan failed on my website. Why?
Usually one of: the page takes too long to load and the test times out; DNS problems prevent the page loading; or the site blocks automated browsers. Check your load time and DNS settings and try again.
What changed from the old CO2 Observer?
- Model: we moved from our own Andrae-based formula (1.805 kWh/GB, 475 g/kWh) to SWDM v4: same lineage, newer data, and comparable with other tools.
- Grades: fixed thresholds instead of a ranking against other scanned sites, so numbers are lower and grades are stable.
- Badge: a plain SVG image that always shows your latest result, with no JavaScript embed.