Measure the Carbon Impact of the Web
Almost all of us use the web daily, and like everything else, we need to make using it more sustainable. SCI for Web extends the ISO-accredited Software Carbon Intensity standard to web applications — covering servers, networks, third-party services, and end-user devices.
What is SCI for Web?
SCI for Web is a domain-specific extension of the SCI specification (ISO/IEC 21031:2024) for measuring the carbon intensity of web applications. A web application is characterised by three essential criteria: content and functionality delivered over HTTP/HTTPS protocols, rendering and execution occurring primarily within web browser environments, and interfaces designed for direct human interaction rather than exclusively machine-to-machine communication.
Why the Web Needs SCI for Web
As the SCI specification matured into an ISO standard, GSF members recognised that a domain-specific, SCI-aligned approach was needed for web applications. Tools and frameworks like the Sustainable Web Design Model, CO2.js, and the W3C Web Sustainability Guidelines have made web emissions visible and established foundational methodologies that thousands of practitioners use. But as organisations move from awareness to accountability, the industry needs approaches that support optimisation decisions across the full delivery chain — including servers, browsers, and third-party services.
The SCI Formula, Applied to the Web
SCI for Web uses the same core formula as the parent SCI standard, but defines web-appropriate boundaries across servers, networks, third parties, and end-user devices — with functional units that reflect delivered web functionality.
SCI = (E × I + M) per R
E — Energy
Energy consumed across the full delivery chain: servers, CDNs, networks, and end-user devices including browsers.
I — Carbon Intensity
The carbon intensity of the energy used, based on location and time — accounting for the global, distributed nature of web delivery.
M — Embodied
Embodied emissions from manufacturing the hardware that serves, delivers, and renders web content.
R — Functional Unit
A web-appropriate functional unit — per page view, per transaction, per session — that reveals true efficiency.
The Measurement Gap
Existing tools have made web emissions visible, but critical gaps remain.
No Standard Boundaries
Different tools measure different things, making results incomparable across organisations.
Third Parties Are Invisible
Analytics, advertising, CDNs, and authentication consume energy on both servers and client devices but are excluded from most measurements.
Totals, Not Rates
Existing approaches measure total emissions rather than carbon per functional unit, making it impossible to reward efficiency.
No Validation
Without mandatory disclosure of boundaries, assumptions, and methods, reduction claims cannot be verified or compared.
How SCI for Web Solves This
SCI provides a strong foundation to measure and reduce web emissions because it is clear about boundaries (what is in scope and what is not), rates (carbon per functional unit, not just totals), and reporting (disclosed methodology so results are comparable and defensible). SCI for Web extends the parent specification to web-specific contexts.
Evaluation Criteria
The assembly agreed on key principles to ensure that optimising SCI for Web scores aligns with genuine carbon reduction.
Accuracy Proportionate to Control
Directly measured components require higher accuracy than modelled estimates. This ensures confident decision-making where practitioners have the most influence.
Mandatory Disclosure
Boundaries, assumptions, data sources, and methods must be clearly stated. This prevents gaming — for example, measurement approaches that automatically credit 'green hosting' based on provider claims allow scores to improve without reducing actual energy use.
Integration Over Complexity
Implementation should fit existing workflows, not require new toolchains. Workflow integration removes adoption barriers, and accurate measurement enables confident decision-making.
A specification that is technically accurate but unused won't serve its purpose, and a widely used metric that lacks credibility also won't serve the purpose.
Built in Collaboration with the W3C
SCI for Web is developed in formal collaboration with the World Wide Web Consortium (W3C). This partnership combines GSF's expertise in software carbon measurement with W3C's global reach in web standards — brokered by the Green Web Foundation, who are active members of both communities. GSF manages the specification through its consensus-driven process, while W3C's Sustainable Web Interest Group provides feedback and insights.
Web Applications In Scope
The scope encompasses the full spectrum of browser-based applications regardless of architectural sophistication. The distinguishing principle is browser-mediated human interaction: if users primarily access functionality through web browsers to accomplish tasks or consume content, the application falls within scope. This definition is intentionally platform-independent and technology-agnostic, focusing on delivery mechanism and user interaction patterns rather than specific implementation technologies.
Static content websites, dynamic platforms, and single-page applications (SPAs)
Server-side rendered applications, e-commerce systems, and media streaming services
Real-time collaborative tools and browser-based management dashboards
APIs accessed primarily through browser interfaces (pure machine-to-machine APIs are out of scope and should use the base SCI methodology)
Built for Every Role in Web Development
Third-Party Transparency
Modern web applications rely heavily on third-party services — analytics, advertising, CDNs, authentication, and payments — that consume energy on both servers and end-user devices. SCI for Web requires these dependencies to be included in the measurement boundary because excluding them hides a significant portion of a website's true carbon footprint. Where precise energy data isn't available from suppliers, practitioners can use industry default values as long as they disclose that estimates were used. The goal is to create market pressure: when organisations start measuring third-party emissions, suppliers have an incentive to publish their own energy data and improve efficiency.
Principles, Not Prescriptions
SCI for Web describes principles and patterns, not prescriptive engineering requirements. Teams should evaluate these within their own context and implement them in ways that suit their architecture and constraints. Crucially, adoption can be incremental: start by measuring the components you directly control — client-side code and server infrastructure — then progressively expand to include network transfer and third-party services. The only requirement is transparency: your disclosed methodology must clearly state which components are measured, which are estimated, and which are excluded.
“Almost all of us use the web daily, and like everything else, we need to make using it more sustainable. By defining a standard for measuring website emissions, we make it easier for people to request greener digital services, for responsible technologists to build them, and to reach the fossil-free internet we all need.”
How We'll Know It's Working
SCI for Web will be successful when it drives real change across the web ecosystem.
Workflow Integration
Developers can integrate SCI for Web into existing workflows without specialised training.
Informed Optimisation
Organisations use measurements to prioritise which optimisations deliver the most carbon reduction.
Third-Party Transparency
Third-party service providers start publishing energy data because customers need it for calculations.
Open-Source Tooling
Multiple open-source tools emerge to make measurement progressively easier.
The Journey to Web Sustainability Measurement
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Aug 2025
Assembly Applications
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Sep 2025
W3C Collaboration
Assembly Starts -
Nov 2025
Assembly Consensus
Design Document -
Q1 2026
Specification Development
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2026
Draft for Review
Get Involved
Help Shape Web Sustainability Measurement
Featured Articles
Reports, announcements, and insights from the SCI for Web journey.
Designing SCI for Web: What We Agreed and What Comes Next
In autumn 2025, a group of GSF members reached consensus on the design foundation for SCI for Web. Here is what they agreed on, how the AI-assisted process worked, and what's next.
The Green Software Foundation and World Wide Web Consortium (W3C) Collaborate to Advance Adoption of Web Sustainability Measurement
The Green Software Foundation and World Wide Web Consortium proudly announce a startegic collaboration to advance and standardize how we measure website carbon emissions.
Pioneering Digital Sustainability in Higher Education—Meet Emma Horrell of the University of Edinburgh
The new GSF member shares how the University of Edinburgh is reducing its digital carbon footprint through practical steps like website measurement, image optimization, and behavioral change initiatives.