WEBSITE SUSTAINABILITY RATING: A TO D EXPLAINED
Every website scan produces an A, B, C, or D sustainability rating based on its CO₂ emissions per page view. Here's what each grade means, what causes it, and how to move up the scale.
Check Your Rating →What is a Website Sustainability Rating?
A website sustainability rating is a letter grade (A through D) that summarises how many grams of CO₂ a website produces per page view. It's derived from measuring total data transferred during a real browser load, then applying the Sustainable Web Design (SWD) model to convert bytes into carbon emissions.
The rating gives web teams, agencies, and sustainability leads a single, communicable number — easy to track over time and easy to include in stakeholder reports alongside performance metrics.
The A–D Rating Scale
A
Excellent
< 0.2g CO₂ per page view
An A-rated site is lean, fast, and producing minimal digital carbon. These sites are typically well-optimized: compressed assets, minimal third-party scripts, and efficient delivery. Less than 10% of websites achieve this rating.
Common examples: Well-optimized marketing sites, lean single-page docs, static sites.
B
Good
0.2g – 0.5g CO₂ per page view
A B rating means the site is above average but has room to improve. Usually characterized by a few heavy images, a standard set of analytics/marketing tags, or a moderately large JavaScript bundle.
Common examples: Standard WordPress sites with some optimization, SaaS marketing pages.
C
Average — Needs Work
0.5g – 1.0g CO₂ per page view
A C rating is the most common. This is where the average website lands. Sites in this range typically have unoptimized images, multiple analytics and retargeting scripts, and heavy font files.
Common examples: Typical ecommerce pages, content-heavy media sites, unoptimized marketing sites.
D
Poor
> 1.0g CO₂ per page view
A D rating indicates a heavy page with significant optimization opportunities. Common causes: autoplay video, large hero images, many third-party scripts, unminified code, and no caching.
Common examples: Rich media landing pages, video-heavy sites, sites with excessive tag manager scripts.
What Factors Affect Your Rating?
Every byte transferred during a page load contributes to the CO₂ score. The biggest contributors are:
Images
Uncompressed or oversized images are often the single largest contributor — a 3MB hero image can easily account for 70% of a page's data transfer.
JavaScript bundles
Unused dependencies, unminified code, and large frameworks loaded on every page add significant weight.
Third-party scripts
Analytics, retargeting pixels, chat widgets, A/B testing tools — each external script triggers its own requests and adds latency.
Video
Autoplay video backgrounds are the fastest route to a D rating. Even a 10-second autoplay clip can transfer 5–20MB per load.
Fonts
Custom webfonts loaded in multiple weights and formats can add 300–800KB to a page.
Hosting and CDN
A CDN reduces latency but the data still transfers. Green hosting powered by renewables reduces the carbon intensity of each byte.
How to Improve Your Website Sustainability Rating
Moving from D → C is usually about removing obvious bloat. Moving from C → B requires deliberate optimization. Moving from B → A requires discipline in what you ship. Here's the priority order:
1. Compress and size images correctly
Convert to WebP or AVIF. Serve responsive sizes with <picture> or srcset. Use lazy loading for below-the-fold images. This alone often moves a C to a B.
2. Audit and remove third-party scripts
Run a tag audit. Remove scripts that aren't actively used. Replace heavy analytics suites with lightweight alternatives.
3. Reduce JavaScript bundle size
Tree-shake unused libraries. Code-split by route. Defer non-critical scripts. Aim for under 100KB initial JS payload.
4. Remove or lazy-load video
Never autoplay video without user interaction. Host video on a CDN with adaptive bitrate streaming.
5. Use system fonts or a single font file
Replace multi-weight custom fonts with the system font stack, or load a single weight with font-display: swap.
6. Track your progress
Optimization without measurement is guesswork. Set up automated daily scans and watch your A–D rating trend over time.
Related Reading
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