Sustainable Web Design Ratings v2.0 explained: how your site gets graded (and 12 fixes that move the needle)

Updated May 2026 · 11 min read · By the Carbon Badge team

Most people discover their website's carbon rating for the first time when someone adds a carbon badge to their site or when a sustainability team asks for Scope 3 emissions data. The letter grade — A+, A, B, C, D, or F — seems clear enough until you try to improve it and realise the model has more nuance than it first appears. Understanding what actually drives the grade is the only way to spend your optimisation effort on things that matter.

The SWD Rating Scale: What Each Grade Means

The scale is based on the distribution of ~50 million pages analysed using the SWD Model v4. The boundaries are not arbitrary — they reflect real-world percentile breakpoints in the dataset, so moving from C to B genuinely means outperforming half of measured websites.

How the SWD Model Calculates Your Score

The Sustainable Web Design model uses three energy components to calculate total CO₂e per page view:

Component 1: Data Transfer Energy

Every byte transferred from server to browser consumes energy across the network infrastructure — routers, switches, last-mile connections, and CDN nodes. The SWD Model v4 uses an energy intensity of approximately 0.06 kWh per gigabyte of data transferred (down from earlier versions, reflecting efficiency improvements in network infrastructure since 2020).

The key insight: this number already accounts for CDN caching. If a CDN delivers your cached assets from edge nodes, the transfer distance — and thus energy per byte — is lower than a request served from a central data centre in another country. Having a CDN configured with aggressive caching can reduce your data transfer energy component by 10–30%.

Component 2: Data Centre Energy

Server-side energy is where the renewable energy mix of your hosting provider has a direct, measurable impact. The SWD model uses a global average grid carbon intensity by default, but if your hosting provider is verified as running on 100% renewable energy by the Green Web Foundation, the model applies a near-zero carbon factor to this component.

This is why switching to green hosting is often the single fastest way to improve your rating by a full letter grade without touching any code.

Component 3: End-User Device Energy

The browser and device rendering your page consume electricity. The SWD model uses a global average device energy intensity (a blend of desktop, laptop, tablet, and mobile consumption patterns, weighted by global usage shares).

Heavy JavaScript pages — those that require significant CPU processing to parse, compile, and execute scripts — have higher device energy consumption than simple HTML pages. This is why a JavaScript-heavy single-page application can have worse carbon credentials than a static HTML site with the same visual result.

The 12 Fixes That Actually Move Your Grade

The following optimisations are ranked by their typical impact on your SWD rating, from highest to lowest. They are based on analysis of 10,000+ sites audited through Carbon Badge.

Tier 1: High impact — often worth 1–2 letter grades

1. Switch to verified green hosting — Reduces the data centre energy component by 70–90%. If your current hosting uses the global average grid mix (carbon intensity ~475 gCO₂/kWh), switching to 100% renewable hosting (verified by Green Web Foundation) can move a C-grade site to B or A. One infrastructure change; no code required.
2. Remove auto-playing video — A single embedded video player loads 1.5–4 MB before the user interacts with it. Replace auto-play with a poster image that loads the player only on click. This is the biggest single-asset change for sites that currently include video backgrounds or hero videos.
3. Eliminate non-essential third-party scripts — The average website loads 15–30 third-party scripts; 5–8 are typically sufficient. Each third-party domain requires a separate DNS lookup, TCP handshake, and HTTP request. Removing unused analytics platforms, dead marketing tools, and duplicate tag manager configurations often cuts page weight by 30–50%.

Tier 2: Medium impact — typically worth half a letter grade

4. Convert images to WebP or AVIF — WebP provides ~30% better compression than JPEG at equivalent quality; AVIF provides ~50% better compression. For a site where images represent 60–80% of page weight (typical for most content sites), this alone often reduces total data transfer by 20–40%.
5. Implement lazy loading — Add loading="lazy" to all images below the fold. Browsers then defer loading those images until the user scrolls toward them. On long content pages, this reduces initial data transfer by 30–60% — most users never scroll to page bottom.
6. Remove unused JavaScript — Open Chrome DevTools → Coverage tab → record a page load. Most sites have 40–60% of their JavaScript never executed on a given page view. Remove unused libraries, dead code from old features, and tools that were trialled and replaced but never removed from the codebase.
7. Use system fonts for body text — Custom web fonts (Google Fonts, Adobe Fonts) add 80–400 KB of data transfer and require external DNS lookups that block rendering. Body text set in -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto loads from the user's device with zero data transfer.

Tier 3: Incremental gains — typically worth 0.05–0.2g CO₂e per page view

8. Implement a CDN with aggressive cache headers — Set Cache-Control: max-age=31536000, immutable on static assets (JS, CSS, images with content-hashed filenames). Returning visitors receive these from browser cache with zero data transfer; new visitors receive them from the nearest CDN edge node rather than your origin server.
9. Reduce DOM size below 1,500 nodes — Large DOM trees (5,000+ nodes) require more browser memory and CPU. This affects device energy consumption — the end-user component of your score. Use Lighthouse's DOM Size audit to identify pages with excessive node counts.
10. Consolidate to a single analytics platform — Many sites run 2–4 analytics tools simultaneously (Google Analytics + Hotjar + Plausible + Facebook Pixel, for example). Each script adds weight and an external request. Choose one analytics platform and remove the rest.
11. Optimise your web font loading strategy — If you cannot switch to system fonts, at minimum: self-host your fonts (eliminates external DNS lookup), use font-display: swap (prevents render-blocking), subset fonts to include only the character sets you use (reduces file size by 50–90% for Latin-only sites), and preload the font files used in your main stylesheet.
12. Upgrade to HTTP/2 or HTTP/3 — HTTP/2 multiplexes requests over a single connection (eliminating connection overhead for parallel assets), and HTTP/3 further reduces latency on mobile connections. This reduces the network energy component of your score, particularly for asset-heavy pages. Check your hosting control panel — most modern providers enable HTTP/2 by default; HTTP/3 requires explicit activation.

What Changes Between Versions of the SWD Model?

The SWD model has evolved from v1 (2020) through v4 (2023). The major changes between versions:

If you measured your site's rating in 2021–2022 using v1 of the model, your current v4 score will likely be better even if you have made no changes — due to the revised network energy intensity. This is methodologically correct: actual network infrastructure has become more energy-efficient. A site that was C+ in 2021 may now be B− for the same page weight.

Sustainable Web Design Ratings and CSRD Scope 3 Reporting

Companies subject to the EU's Corporate Sustainability Reporting Directive (CSRD) must disclose Scope 3 emissions from fiscal year 2025 onwards. Digital infrastructure — including websites and cloud services — falls under Scope 3 Category 1 (purchased goods and services) and Category 11 (use of sold products, for technology companies whose products are websites or apps).

The SWD Model v4 is an accepted methodology for this calculation. Carbon Badge generates a CO₂e estimate per page view and per month (based on your traffic volume) using the SWD model, which can be cited in sustainability reports with the methodology reference. If you need a more rigorous audit for CSRD purposes, supplement the automated calculation with a manual energy audit of your server infrastructure.

Related: CSRD Scope 3 and website emissions reporting 2026 · Website carbon benchmarks by industry 2026 · Best green web hosting providers 2026

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