Website Carbon Calculator

A HAR file is the page exactly as your browser loaded it: every request, its size, and which site sent it, including pages behind a sign-in or not yet public. It is read in your browser, so nothing about the page leaves your computer, and there is no crawler for a site to block.

By the Sustainable Web Design Model, version 4, the method behind CO2.js and most website carbon tools: the data a page view moves, times 0.3 kWh per GB across data centers, networks and visitors’ devices, running and making them, times the grid. Every visit is counted as a first visit, as CO2.js does.

The model’s digital carbon ratings, from A+ (0.040 g or less per page view) to F (0.360 g or more), set from the HTTP Archive’s crawl of the web in June 2023, so a grade compares your page with the web as it was then.

Here, yes: it is read by JavaScript in this browser and this page has no way to upload it. But a HAR file holds the cookies and sign-in tokens of the visit it recorded, so record a signed-out visit in a private window, and never send the file to anyone.

Start with the heaviest requests: images sized for the screen and served as WebP or AVIF, video that loads only when played, fewer fonts, and less JavaScript, especially from other sites. Lighter pages also load faster.