The underlying model
We use the Sustainable Web Design Model (SWDM v4) from the Green Web Foundation. It weighs data transfer, end-device energy, network energy and data-centre energy. Simplified:
1 GB of transferred data ≈ 0.81 kWh ≈ 0.36 kg CO₂ (global grid mix). With an average ~2 MB per visit, that's 0.72 g CO₂.
When hosting runs on verifiably green power (Cloudflare since 2022, Google Cloud since 2017), the data-centre share is typically reduced to around 30 % of the total.
What we measure
- HTML size of the main page (gzip-compressed)
- Asset sizes of the top 10 images, videos, scripts and fonts
- Hosting provider: via reverse DNS or WHOIS, matched against the Green Web Foundation list
- Cache headers (Lighthouse audit): returning visitors pay less
- Codec choice for video (AV1 vs H.264 vs VP9)
- Image format (AVIF vs WebP vs JPG/PNG)
What we can't measure
Honesty is part of the brand. These factors influence the real footprint but cannot be measured from the outside:
- Server-side energy efficiency (PUE) of the actual data centre
- Device type of the visitor (smartphone vs desktop consume differently)
- Geographical distribution of visitors (grid mix varies by country)
- Mobile network vs Wi-Fi share (cellular uses ~3× more energy)
We estimate with the global average and flag that transparently.
Our measurement tiers
- Tier 1, quick check (audit tool, free). main page plus top 10 assets. Estimate with ±30 % uncertainty. Result in ~5 seconds.
- Tier 2, full-site audit (290 €). crawl of all indexable pages up to 200 URLs. Lighthouse audits on 5 representative pages. ±15 % uncertainty.
- Tier 3, CSRD report (recurring). quarterly measurement, statistical analysis over time, audit-ready documentation. ±10 %.
Sources
Sustainable Web Design Model v4 · sustainablewebdesign.org/calculating-digital-emissions
Green Web Foundation · thegreenwebfoundation.org
websitecarbon.com methodology · websitecarbon.com/how-does-it-work
Cloudflare Sustainability Report · cloudflare.com/impact
CSRD Directive EU 2022/2464 · eur-lex.europa.eu