Website Impact Optimisation — Methodology & Sources
Built by Forest Web Hosting · theforest.uk
Version: 1.0.0 · Date: June 2026
Carbon Calculation — SWDM v4
The core carbon formula is based on the Sustainable Web Design Model (SWDM) v4, an open methodology published by Wholegrain Digital and the Sustainable Web Design community, designed specifically for tools like this to use freely.
Reference: sustainablewebdesign.org
Formula
CO₂e (grams) = Page size (GB) × 0.81 kWh/GB × 494 gCO₂e/kWhFor green-hosted sites, the grid intensity is reduced by 85%:
CO₂e (grams) = Page size (GB) × 0.81 kWh/GB × (494 × 0.15) gCO₂e/kWhConstants
| Constant | Value | Source |
|---|---|---|
| Energy per GB transferred | 0.81 kWh/GB | SWDM v4 |
| Global grid intensity | 494 gCO₂e/kWh | SWDM v4 (global average) |
| Green hosting reduction | 85% (factor 0.15) | SWDM v4 |
Green Hosting Check — Green Web Foundation
Whether a website is hosted on renewable energy is determined by the Green Web Foundation API, a free public API maintained by the Green Web Foundation and designed to be used by third-party tools.
Reference: thegreenwebfoundation.org
API endpoint: https://api.thegreenwebfoundation.org/api/v3/greencheck/{hostname}
Only the hostname is sent — no full URLs or browsing data leave the browser.
Regional Grid Averages
Percentile ranking compares the scanned page against regional averages using a log-normal distribution model.
| Region | Average CO₂e per page view | Source |
|---|---|---|
| Global | 0.36g | SWDM v4 / HTTP Archive |
| UK | 0.28g | SWDM v4 regional data |
| US | 0.32g | SWDM v4 regional data |
| EU | 0.22g | SWDM v4 regional data |
Region is detected from navigator.language — no IP geolocation used.
Equivalents — Emission Factors
All equivalents use publicly available, peer-reviewed emission factors.
☕ Coffee (21g CO₂e per cup)
Includes production, roasting and brewing. Based on lifecycle assessment data widely cited in food carbon literature.
Source: Published lifecycle assessment research for Arabica coffee.
✈️ Flying economy (255g CO₂e per passenger mile)
Long-haul economy class, including radiative forcing uplift factor.
Source: UK DEFRA/DESNZ Greenhouse Gas Conversion Factors for Company Reporting (2023).
Reference: gov.uk — Greenhouse Gas Conversion Factors
🚗 Petrol car (274g CO₂e per mile)
Average UK petrol car, including fuel production (well-to-wheel).
Source: UK DEFRA/DESNZ Greenhouse Gas Conversion Factors for Company Reporting (2023).
Reference: gov.uk — Greenhouse Gas Conversion Factors
📱 Phone charge (1.92g CO₂e per full charge)
Based on a typical smartphone battery (~15 Wh) charged on the UK grid.
Source: DEFRA UK grid intensity combined with typical smartphone battery capacity.
🎤 Netflix streaming (36g CO₂e per hour)
Carbon footprint of one hour of streaming video, including data centre and network energy.
Source: Carbon Trust / IEA streaming media analysis, widely adopted figure in digital carbon research.
🌳 Tree absorption (60g CO₂e per day)
Daily CO₂ absorption rate of a young/average deciduous tree.
Source: US Forest Service research; figure consistent with UK Woodland Carbon Code guidance.
A–F Impact Rating Scale
The rating is WIO’s own original methodology, calibrated against the SWDM v4 thresholds.
| Grade | CO₂e per page view | Label |
|---|---|---|
| A+ | < 0.095g | Exceptional |
| A | < 0.185g | Excellent |
| B | < 0.360g | Good |
| C | < 0.540g | Average |
| D | < 0.720g | Poor |
| E | < 1.000g | Very Poor |
| F | ≥ 1.000g | Failing |
The B/C boundary is set at the global average (0.36g) so a B grade means better than average.
Technical Audit Scoring
The seven-element technical audit and its composite Tech Health score are original WIO methodology — not derived from any third-party scoring system.
| Element | Weight | Pass threshold |
|---|---|---|
| Page weight | 20% | < 500 KB |
| Image formats (WebP/AVIF) | 15% | ≥ 80% modern format |
| Lazy loading | 10% | ≥ 80% lazy loaded |
| Third-party scripts | 15% | 0–2 external scripts |
| Render blocking scripts | 10% | 0 blocking scripts |
| Caching | 15% | ≥ 60% resources cached |
| Dark mode support | 5% | prefers-color-scheme detected |
Page Weight Classification
Benchmarked against HTTP Archive global page weight data (2024).
| Badge | Threshold | Context |
|---|---|---|
| Light | < 500 KB | Well optimised |
| Moderate | < 1 MB | Good |
| Average | < 2.5 MB | Near global average (~2.5 MB) |
| Heavy | < 5 MB | Above average |
| Very heavy | ≥ 5 MB | Significantly above average |
Source: HTTP Archive Web Almanac
Page Resource Measurement
All page data is measured locally in the browser using the Performance API (performance.getEntriesByType('resource') and performance.getEntriesByType('navigation')). This captures the actual bytes transferred for all resources on the page.
No data is sent to theforest.uk servers. The only external call is to the Green Web Foundation API (hostname only).
Privacy
- No browsing data stored
- No URLs tracked or logged
- No analytics or telemetry
- Session data (page count, total grams) is in-memory only — cleared when the popup closes
- GWF check: hostname only, never full URLs
Attribution Summary
| Source | Used for | Licence |
|---|---|---|
| SWDM v4 | Core carbon formula and constants | Open, free to use |
| Green Web Foundation API | Green hosting detection | Free public API |
| UK DEFRA Conversion Factors 2023 | Flight and car emission factors | Open Government Licence |
| HTTP Archive Web Almanac 2024 | Page weight benchmarks | Creative Commons |
| US Forest Service / Woodland Carbon Code | Tree absorption rate | Public domain |
Website Impact Optimisation v1.0.0 · Built by Forest Web Hosting