What does it really mean to call a website sustainable? We break down practical benchmarks, common pitfalls, and the decisions that genuinely reduce a website’s carbon footprint.
We are often asked what qualifies a website as “sustainable”, and what figure is needed to use that label with confidence. Many organisations are cautious, concerned about making claims that could be challenged as greenwashing. Others simply do not know what benchmark they should be aiming for, because there is no widely agreed standard.
This uncertainty exists because web sustainability lacks clear, universally accepted thresholds. Unlike accessibility or performance, there is no single pass or fail measure, and different tools assess impact in different ways.
We have written previously about the lack of benchmarks and the idea of a digital thermostat for websites. The principle is straightforward: without a starting point, it is difficult to judge progress or make informed decisions. A reference figure gives organisations something to work towards and something to assess changes against over time.
Based on our experience designing and developing websites, we believe a useful starting point is less than 0.5g of CO₂ per page load. A website performing at this level has typically been optimised to reduce unnecessary data transfer and processing, resulting in a more lightweight, lower-carbon experience. Simpler websites can and should be lower, but this figure provides a practical baseline for understanding what is generally considered good versus poor performance. Context is crucial, as it is not necessarily meaningful to directly compare a large, complex website such as a university with a small service-based site, such as a piano teacher advertising their services.
Existing grading systems highlight part of the challenge. For example, the Sustainable Web Design website grades pages emitting more than 0.36g of CO₂ as an F, yet the site itself reports emissions of 0.49g per page. Whether this figure represents a single page or an average across the site is unclear, and no further detail is provided.
This lack of clarity is common. Different tools use different assumptions and methodologies, and entering the same URL into multiple auditors often produces different results. While there has been some effort to align approaches across organisations, there is still no unanimous or fully transparent standard.
This is one of the reasons we have built our own web sustainability tool, with a public launch coming soon. Our aim is to provide a method we can rely on for our clients, and one that goes further than many existing tools in terms of what is measured and how results are interpreted.
What makes a website sustainable in practice?
There is no single action that makes a website sustainable. Instead, sustainability comes from a set of decisions that reduce energy use every time a page is loaded. The list of recommendations below is not exhaustive, but addressing these will make a measurable difference to a website’s carbon load and how sustainable it can reasonably be considered.
Lightweight page weight
The size of a webpage directly affects how much energy is required to load it. Fewer and smaller assets mean less data transferred across networks and less processing on servers and devices. Keeping page weight low has an immediate and repeatable impact at scale.
Efficient code and architecture
Inefficient or bloated code increases processing time and energy use on both the server and the user’s device. Clean, well-structured code and minimal dependencies reduce this overhead. This also improves performance and makes the site easier to maintain long-term.
Considered use of media
Images and video are often the largest contributors to a page’s carbon footprint. Serving media at appropriate sizes, compressing files, and avoiding unnecessary or autoplaying video significantly reduces data transfer. Allowing users to choose when heavier content loads is usually both more sustainable and better for user experience.
Hosting and infrastructure choices
The environmental impact of a website is influenced by where and how it is hosted. Using providers powered by renewable or low-carbon energy and implementing efficient caching reduces unnecessary energy use.
Longevity and ongoing maintenance
Websites tend to become heavier over time as content, plugins, and features are added. Clear guidelines and regular reviews help prevent gradual efficiency loss. A site that is monitored and maintained will remain more sustainable than one optimised only at launch.
In summary
What makes a website sustainable is not a single metric or label. It is the combination of conscious design and development decisions, supported by measurement, and maintained over time.
Rather than trying to answer with a yes/no as to whether a website is sustainable, a more useful measurement is to assess whether clear steps have been taken to reduce its carbon load, and whether those steps are being monitored and upheld as the site evolves.
