How CO2Stats works

A check loads one page once, in a real browser, under a fixed test profile (profile m1): a desktop window of 1366 by 768 pixels, an empty cache, no cookies and no consent banners clicked. The browser scrolls to the bottom so that lazy-loaded content appears, then waits until nothing has arrived for 2 seconds, or for at most 30 seconds from the start.

Transfer size is every byte the browser receives in that time, compressed as the server sent it, including files from other websites.

A result is incomplete if the page ran out of time, reached a limit, showed a challenge or an error, or at least 10 requests failed and they were 20% or more of all requests.

• A check stops at 300 requests, or once the transfer it has counted passes 25 MB, and follows at most 5 redirects.
• The same page is measured afresh at most once every 24 hours; until then the stored result is shown.
• There is a daily budget for checks. When it runs out, the checker asks you to try again later.

Grams of CO2e per visit come from @tgwf/co2 version 0.19.0, using the Sustainable Web Design Model v4 (perVisit, with its default mix of first and return visits; carbon model c1). If the page's own host is verified green by the Green Web Foundation, the whole page counts as green-hosted. The grid figure for the server's country is shown for context and does not change the grams.

The grade comes from the Sustainable Web Design rating table, applied to the grams. Each limit includes its own value.

The first line divides a page’s grams by grade A’s limit, 0.079 g per visit.

The second compares them with boiling the kettle for a cup of tea: heating one cup’s water (250 mL, 15 to 100 °C) takes 24.7 Wh before any kettle’s losses, which on the UK’s average grid (218 g CO2e per kWh in 2025, from Ember) gives off 5.4 g of CO2e.

A result lists what would make the page lighter, each with its own saving in grams per visit. The savings overlap, so they are never added together.

A heavy file: any file that is at least 10% of the page's transfer and at least 50 KB, up to 5 files. Its saving is what the page would save without that file at all.

A green host: when the page's host is not verified green, the saving is what the same page would save on a verified green host.

A result shows the provider that delivered the page, where known, and the country its server appears to be in. Behind a content delivery network, that country may be the network's, not the origin's.

"Verified green" means the Green Web Foundation listed the host as verified on the date we checked. Since October 2026 the Foundation no longer accepts carbon offsets as evidence; hosts verified earlier are checked against the new rules when they renew.

The grid figure is the average for the countries where a host has data centres, from Ember’s yearly electricity data. A host with no grid figure comes last in its group; ties go by name.

CO2Stats Verified describes one page, not a whole website. A page earns it with a complete check, a host verified green and grade A+, A, B. Ask for it from the page's result.

A certificate lasts 45 days from the check it rests on. Any later complete check of the page counts, whoever runs it: one that passes renews the certificate, and one that fails ends it at once. We re-check certified pages 30 days after their last check.

The badge shows the page, its grade, its grams and the date of the check. Browsers ask for it again each time it is shown, so a certificate that ends shows on the next view.

This page describes measurement profile m1, carbon model c1 and improvement rules i1. Every result records the profile and carbon model it used.