How does the carbon footprint checker calculate emissions?
We fetch the page once with a real browser User-Agent, follow redirects, and measure how many bytes the server actually returns. Then we apply the model popularised by Wholegrain Digital and CO2.js: bytes are converted to gigabytes, multiplied by 0.81 kWh/GB (the global average energy cost of moving data from data centre to device), and that kWh figure is multiplied by 442 g/kWh - the worldwide average grid intensity in 2023. The result is grams of CO₂ per single visit. The "green hosting" number swaps 442 g/kWh for 50 g/kWh, the typical figure for a renewables-powered data centre.
Why is the size I see different from what Chrome DevTools shows?
DevTools shows the size of every sub-resource the browser loaded - HTML, scripts, images, fonts, third-party tracking, and so on. Our checker only fetches the HTML document itself, so it under-estimates total weight for image-heavy or script-heavy pages. The number is still useful as a relative measure across pages on the same site, but treat it as a floor, not a ceiling. For an exhaustive measurement, use a tool that runs a full headless browser like Lighthouse or WebPageTest.
What counts as a "good" carbon score?
Under 0.5 g of CO₂ per visit is "very green" - typical for a well-optimised text-and-images blog post. 0.5-1 g is "good", which most modern marketing sites achieve. 1-2 g is "average". 2-5 g is "high" and usually means heavy hero videos, autoplay media, or a lot of third-party scripts. Above 5 g is "very high" and worth investigating: every 10,000 visits at 5 g = 50 kg of CO₂, which adds up across a year.
Is the energy figure (0.81 kWh/GB) accurate?
It is a widely used industry estimate but not a measurement of your specific page. The 0.81 kWh/GB figure comes from the Sustainable Web Design model and represents the entire chain - data centre power, network transit, end-user device. Newer research suggests the real figure may be lower (closer to 0.06 kWh/GB for transit alone), but 0.81 stays popular because it captures device energy too. Use the score for direction, not for offset purchasing.
Does HTTPS, caching, or CDN make a difference?
Yes for second-time visitors, no for first visits. Our tool measures the worst case: a fresh visitor who has nothing in cache. Real-world averages are lower because returning users hit the browser cache and skip most asset transfers. CDNs reduce per-byte energy slightly by serving from a closer edge, but the dominant factor is still total bytes. The fastest carbon win is almost always image compression and removing unused JavaScript.
How can I lower my page's carbon score?
Compress images aggressively (WebP or AVIF, lazy-load anything below the fold), defer non-critical scripts, ditch the autoplay video hero, audit third-party tags (analytics, chat widgets, ad pixels often add 200-500 KB each), and host on a green provider. Going from 5 MB to 1 MB on a high-traffic homepage typically cuts emissions by 80% and improves Core Web Vitals at the same time, so it's a free SEO win on top.
