Our daily work as digital professionals has a measurable impact on the environment. Understanding digital carbon ratings can help you make your site more sustainable.
As digital professionals, we’re increasingly aware that the work we do has real environmental consequences. The carbon cost of the internet is often overlooked, but it’s not insignificant. The web is powered by electricity, and a lot of that electricity still comes from fossil fuels. According to the Sustainable Web Design Group, the internet currently consumes approximately 3.7% of global carbon emissions. And as digital demand grows, that number is expected to rise.
But awareness alone doesn’t lead to change—we need better ways to measure and talk about impact if we want to make meaningful progress. This is where digital carbon ratings come in.
What Are Digital Carbon Ratings?
A digital carbon rating estimates the environmental impact of a website or digital product—specifically, the amount of carbon dioxide (CO₂) emitted when someone loads or interacts with it. Just like physical products have energy ratings, websites can be rated based on the energy required to deliver their content, the efficiency of their code, and the type of energy powering the servers that host them.
Every file downloaded, image loaded, or API call made contributes to emissions. When you consider how much time we all spend online, those small interactions add up quickly. To quantify these impacts, organizations like the Sustainable Web Design Group have developed models for calculating digital emissions and turned them into more easily understandable digital carbon ratings.
These calculations help surface important questions: How efficient is our site? How much data are we moving? What kind of energy is powering our hosting? And more broadly—how can we make better, lower-impact choices in our work?
Digital carbon ratings are intentionally aggressive to push the industry toward better standards
Key Factors Influencing Carbon Ratings
These models estimate the energy required to load a webpage and apply an emissions factor based on the likely carbon intensity of the electricity used. They aim to answer two key questions:
How much energy does this page use?
How carbon-intensive is that energy?
To calculate this, tools consider a few critical factors:
Page weight / data transfer size: Heavier pages consume more energy to transmit.
Number of requests: Each image, font, script, or third-party service adds load.
Hosting provider: Whether the site uses green hosting, as tracked by datasets like the Green Web Foundation’s.
A site’s grade is determined by how its data transfer compares to global averages, using benchmark data from the HTTP Archive, which tracks real-world performance metrics across millions of websites. Tools like the Website Carbon Calculator and Ecograder use this data to define thresholds for each grade.
As of late 2024, the median page weight was 2.3 MB for mobile and 2.65 MB for desktop, according to the Web Almanac. However, these tools often use more stringent thresholds than the median, meaning even average-performing sites may receive low grades.
This is by design. Digital carbon ratings are intentionally aggressive to push the industry toward better standards. The idea isn’t to shame “average” websites, but to highlight how much room there is for improvement—and how common it still is to deliver unnecessarily heavy, resource-intensive experiences. By setting the bar high, these tools encourage teams to optimize not just for performance, but for sustainability too.
Why Detailed Impact Data Matters
While carbon ratings offer a useful overview, it’s crucial to understand how changes to data transfer directly impact emissions. Knowing that a website has a carbon footprint is a good start, but the real value comes when we can break that impact down and connect it to specific design and development decisions. For digital professionals, carbon ratings are not just benchmarks—they’re tools for evaluating the environmental cost of our choices.
Optimizing performance, reducing page weight, and cleaning up unused code not only improve speed and SEO but also reduce energy consumption. Similarly, choosing green hosting, reconsidering auto-playing videos, or simplifying complex user flows can all lower emissions. Carbon ratings help us understand how these everyday decisions add up.
For instance, switching large, high-resolution images to more efficient formats like WebP can reduce image size by up to 80%. Optimizing JavaScript and CSS—by removing unused code, reducing requests, or combining files—also cuts down on data transfer and energy use. Even switching from a complex font to a simpler system font can result in a 20–30% reduction in file size, improving both speed and carbon efficiency.
Additionally, hosting on servers powered by renewable energy can cut emissions by up to 80%, according to the Green Web Foundation. Carbon ratings show you where your site’s emissions come from—whether it’s images, JavaScript, or third-party services—and provide a clear roadmap for making impactful changes.
Example: Reducing Emissions on a Marketing Website
To put the impact of these changes into perspective, let’s take a look at an example using a website that is roughly the average of 2.4 MB in size. A site like this is likely to include several high-resolution images and a few third-party scripts for analytics and social media embeds.
According to the Sustainable Design Group, a 2.4 MB page served from a standard data center using fossil fuel energy might generate around 0.359 grams of CO₂ per page view If the site gets 100,000 monthly views, that’s roughly 35.9 kg of CO₂ per month—equivalent to driving about 89 miles in a typical gasoline car.
If we make some simple changes to the way the website is developed, we can easily have a meaningful impact on the site’s carbon footprint.
Replace JPEGs with WebP: WebP images are typically 25%–34% smaller than JPEGs at equivalent quality. If the median webpage serves around 1052kb of image assets converting to a new format could save approximately 315KB per page load.
Eliminate unused JavaScript and defer third-party scripts: The median desktop website requests about 684KB of Javascript which is far above the target of around 365KB for a 3 second page load. By removing unnecessary code and deferring non-essential scripts, we could reduce data transfer by approximately 320 KB.
These changes reduce the estimated page weight from 2.4 MB to approximately 1.78 MB. This, in turn, reduces emissions per page view from 0.359 grams to about 0.262 grams—a 26% reduction in carbon pollution.
If these optimizations are made across multiple pages—or across multiple client sites—the cumulative impact can quickly multiply. Scaling these changes across 15 websites could save over 1.74 metric tons of CO₂ per year.
Keep in mind that this is the median, which means that 50% of websites have a heavier page weight and a lot more room to make changes that will benefit the environment. And there are many more improvements to consider that would add to the impact—like optimizing web fonts through subsetting and compression, reducing CSS bloat, lazy-loading offscreen images, and self-hosting fewer or more efficient third-party scripts.
The more visibility we have into the environmental cost of the web, the more pressure there is—on ourselves, on teams, on clients, and on the industry—to make better choices.
The Benefits of Understanding Carbon Impact
The more visibility we have into the environmental cost of the web, the more pressure there is—on ourselves, on teams, on clients, and on the industry—to make better choices. Ratings aren’t the final answer, but they help normalize the idea that digital work has a footprint. And once something is measurable, it’s easier to make progress.
Moreover, many of the same practices that reduce carbon emissions also result in faster load times, better accessibility, and a more inclusive experience for users on low-end devices or slower networks. In that way, sustainability becomes part of what it means to build a high-quality digital product.
Over time, the ripple effects of that progress add up: lower energy use across infrastructure, fewer unnecessary requests, more thoughtful design decisions, and ultimately, a web that’s lighter on the planet and better for the people who use it.
The next step isn’t perfection—it’s iteration. The more we treat digital sustainability as something we can measure, test, and improve over time, the more it becomes integrated into how we build. That shift—from awareness to action—is where real change happens.
