Website Carbon Footprint & Page Carbon Rating CO2 Estimator

Carbon footprint of a web page: method, factors and example

The tool converts page weight into energy (0.81 kWh per GB transferred) and then into CO₂ (442 g per kWh, or 210 g with green hosting), following a simplified version of the Sustainable Web Design model. A 1.46 MB page viewed 45,000 times a month thus emits about 0.19 g of CO₂ per visit and 102 kg a year with a green host.

The calculation

• Transferred weight = total weight × (1 − share of returning visitors × 0.75): the tool assumes a returning visitor reloads 25% of the page thanks to caching.
• Energy (kWh) = weight in GB (1 GB = 1,024³ bytes) × 0.81.
• CO₂ (g) = energy × 442, or × 210 when hosting is marked as green.
• Yearly = CO₂ per visit × monthly page views × 12.
• Grade: A+ ≤ 0.095 g, A ≤ 0.186 g, B ≤ 0.341 g, C ≤ 0.493 g, D ≤ 0.656 g, E ≤ 0.846 g, F above.

Worked example (default values)

HTML 35 KB, CSS 65, JavaScript 380, images 850, fonts 120, other 50: 1,500 KB (1.46 MB). With 30% returning visitors, 1,162.5 KB are transferred, i.e. 0.000898 kWh. Green hosting: 0.189 g per visit (grade B) and 101.8 kg a year for 540,000 visits, offset by 5 trees. Without green hosting: 0.397 g (grade C) and 214.3 kg a year.

Limits

• The 0.81 kWh/GB and 442 g/kWh factors belong to version 3 of the Sustainable Web Design model; they are shown as indicative because version 3 is no longer published. Version 4 splits energy by segment (data centres, networks, devices), adds embodied emissions and uses a 494 gCO₂e/kWh global average.
• The 210 g/kWh green-hosting figure is specific to the tool: in the model, green hosting only reduces data-centre emissions.
• The grade scale is the version 3 one, as published in the Green Web Foundation's CO2.js library (SWDMV3_RATINGS); version 4 thresholds are much stricter (A+ ≤ 0.040 g). This grade is also unrelated to GreenIT's EcoIndex, which relies on DOM size and request count.
• Indicative, unsourced equivalents: 22 kg of CO₂ per tree per year, 120 g per km by car, 8.3 g per smartphone charge.

Sources and dates: Sustainable Web Design, Estimating Digital Emissions (version 4); Sustainable Web Design, Digital Carbon Ratings; the tool uses version 3 factors (checked 10/14/2026); grade scale: Green Web Foundation, CO2.js, SWDMV3_RATINGS (checked 10/15/2026).

Frequently Asked Questions

How much CO₂ does a 2 MB web page emit?

With the tool's factors (0.81 kWh per GB and 442 g of CO₂ per kWh), 2 MB transferred is about 0.70 g per visit without caching. Green hosting brings it down to about 0.33 g in the tool.

Which part of page weight matters most for its footprint?

The calculation is proportional to weight: every KB counts the same, whether images or JavaScript. In the default example, images (850 KB) and JavaScript (380 KB) make up 82% of the 1,500 KB.

Does returning-visitor caching really lower the footprint?

Yes in this tool: it assumes a returning visitor downloads only 25% of the page again. With 30% returning visitors, transferred weight drops by 22.5%.

Is this grade the same as the EcoIndex?

No. GreenIT's EcoIndex combines DOM size, request count and page weight, while this grade only depends on estimated CO₂ per visit, on an A+ to F scale taken from the Sustainable Web Design model.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.