What Is carbon.txt and Does Your Website Need One?

There's a new file appearing at the root of forward-thinking websites. It's called carbon.txt, and it's the Green Web Foundation's answer to a straightforward problem: how do you publish credible, machine-readable sustainability data about your digital infrastructure in a place where both humans and software can find it?

If you're familiar with robots.txt or security.txt, the concept will feel familiar. A plain text file at a predictable URL (/carbon.txt) that provides structured information about your organisation's sustainability claims, certifications, and hosting arrangements.

What carbon.txt Actually Is

carbon.txt is a specification maintained by the Green Web Foundation. It defines a standard location and format for publishing sustainability disclosures related to your website's digital infrastructure. The current version (v0.4) uses a TOML-based syntax that allows organisations to declare:

• Upstream providers — Which hosting companies, CDNs, and cloud platforms you use
• Sustainability credentials — Certifications, reports, and evidence that back up green claims
• Validity dates — When each disclosure expires, so downstream tools can discount stale data
• Documentation links — URLs pointing to the actual certificates, annual reports, or third-party audits that substantiate your claims

The key insight behind carbon.txt is that sustainability claims need to be verifiable, not just stated. Anyone can write "we use green hosting" on their about page. carbon.txt asks you to back that up with machine-readable references to actual evidence.

How It Differs from carbon.txt, llms.txt, and robots.txt

The web is accumulating a useful collection of root-level text files, each serving a different purpose:

• robots.txt — Tells search engines what to crawl (since 1994)
• security.txt — Tells security researchers how to report vulnerabilities (RFC 9116)
• llms.txt — Tells AI systems how to understand and represent your organisation
• carbon.txt — Tells automated tools about your infrastructure's sustainability credentials

While llms.txt focuses on how AI systems should interpret your content, carbon.txt focuses on the sustainability credentials of your infrastructure. They're complementary, not competing.

What a carbon.txt File Looks Like

Here's a simplified example of what a carbon.txt file contains:

[upstream]
# Your hosting provider
[[upstream.providers]]
domain = "your-hosting-provider.com"

[upstream.providers.credentials]
type = "annual-report"
url = "https://provider.com/sustainability-report-2025.pdf"
valid_until = "2026-12-31"

[org]
# Your organisation's own disclosures
[[org.credentials]]
domain = "yourdomain.com"
type = "csrd-report"
url = "https://yourdomain.com/sustainability-report.pdf"
valid_until = "2026-12-31"

The syntax is deliberately structured for machine parsing. Tools like the Green Web Foundation's carbon.txt validator can read this file, verify the links, and assess whether your sustainability claims are current and documented.

Why It Matters for UK Organisations

Three converging pressures make carbon.txt increasingly relevant:

1. Regulatory reporting requirements are expanding

The EU's Corporate Sustainability Reporting Directive (CSRD) and the UK's evolving Sustainability Disclosure Requirements are pushing more organisations to report on their environmental impact, including digital infrastructure. carbon.txt provides a standardised way to publish that data at the domain level.

2. Supply chain transparency is expected

Large organisations increasingly need to verify the sustainability credentials of their suppliers. If your hosting provider claims to run on renewable energy, carbon.txt gives you a machine-readable way to reference and verify that claim. It turns a marketing assertion into auditable data.

3. Green hosting claims need substantiation

As we've written about in our article on the reality of green hosting, many sustainability claims in the hosting industry are based on certificate trading rather than genuine emissions reduction. carbon.txt doesn't solve that problem, but it does create a framework where claims must be linked to actual evidence, which makes scrutiny easier.

Should Your Organisation Adopt carbon.txt?

The honest answer: it depends on where you are in your sustainability journey.

You should consider adopting carbon.txt if:

• You already publish sustainability reports or CSRD disclosures
• Your hosting provider has verifiable green credentials you can reference
• You are in a regulated industry where environmental reporting is required or expected
• You want to differentiate from competitors who make unsubstantiated green claims
• You are a B Corp or sustainability-led organisation where credibility matters

You might hold off if:

• You don't currently have sustainability documentation to reference
• Your hosting provider doesn't publish verifiable green credentials
• You're still working on reducing your actual digital emissions (focus on the substance first)

The important principle here is that carbon.txt is a transparency mechanism, not a greenwashing tool. Publishing a carbon.txt file that links to vague marketing pages rather than genuine evidence is worse than not having one at all. The specification is designed for organisations that have real credentials to share.

Where carbon.txt Fits in a Broader Strategy

At OYNK, we see carbon.txt as one component of a broader digital sustainability strategy. The hierarchy of priorities is:

1. Reduce actual emissions first — Optimise page weight, cut unnecessary scripts, minimise data transfer. This is the work that actually changes the number of watts your site draws from the grid.
2. Measure and audit — Use tools like EcoPigs and the P.E.E.R. framework to benchmark your performance with real data.
3. Document and disclose — Once you have genuine improvements to report, carbon.txt gives you a standardised way to publish that evidence.

Publishing a carbon.txt file on a bloated, poorly optimised website is performative. Publishing one on a site that has genuinely been engineered for efficiency is credible.

For organisations working towards this, our Digital Waste Audit is a practical starting point. It gives you the baseline data you need before you can credibly document anything, and the CLEAR framework ensures your site is structured for both human and machine interpretation.

Sustainability reporting is moving from optional to expected. carbon.txt is how that reporting arrives at the domain level. The organisations that adopt it early — with genuine evidence behind it — will have a credibility advantage when it matters most.