web.mit.edu

The numbers

First visit

1.30 g CO₂e

Return visit

1.12 g CO₂e

Hosting

unknown

LCP

548 ms

CLS

0.001

TTFB

15 ms

At 10,000 views a month this page emits about 150.1 kg CO₂e a year, roughly 615 km in an average petrol car. The estimate assumes 75% new and 25% returning visitors, a measured cache reload ratio of 86%, and world-average grid intensity. How we calculate this

1. Replace heavyweight GIFssave ~0.917 g/visit (6.41 MB)High impact · quick fix

   1 GIF totals 7.54 MB. GIF is the least efficient image format still in common use, animated or not.

   If it animates, re-encode the clip as a muted looping MP4 or WebM (typically a 10x reduction); if it is a still, convert it to WebP or AVIF.

   • MIT-Cancer-Models-SL.gif7.54 MB

   WSG 2.9: Optimize media to reduce resource use
2. Fix caching for returning visitorssave ~0.279 g/visit (1.88 MB)High impact · quick fix

   On a repeat visit 7.54 MB of static assets were re-downloaded instead of coming from cache.

   Serve static assets with Cache-Control: public, max-age=31536000, immutable and versioned filenames so returning visitors download almost nothing.

   • MIT-Cancer-Models-SL.gifCache-Control: public, max-age=2531, s-maxage=3600 · 7.54 MB

   WSG 4.2: Optimize caching and support offline access
3. Enable text compressionsave ~0.038 g/visit (263 kB)High impact · quick fix

   1 text resource (376 kB) are served without gzip/Brotli.

   Enable Brotli (or at least gzip) on the server or CDN for HTML, CSS, JavaScript, JSON and SVG. Typical reduction is ~70%.

   • main.js376 kB

   WSG 4.3: Reduce data transfer with compression
4. Convert and compress imagessave ~0.030 g/visit (209 kB)Medium impact · quick fix

   6 images (464 kB) ship in legacy formats (JPEG/PNG/GIF).

   Serve AVIF (or WebP) with responsive srcset sizes, and re-compress oversized files. AVIF typically halves JPEG weight at the same visual quality.

   • 54884213284_1ab5ed6ceb_b_0.jpg145 kB
   • MIT-Lauren-Fortier-SLsub.jpg94 kB
   • MIT-This-is-MIT-SLsub_0_0.jpg87 kB
   • MIT-aerial-3-EmilyDahl_0d.jpg50 kB
   • MIT-Hugh-Smith-01-thumb.jpg46 kB

   WSG 2.9: Optimize media to reduce resource use
5. Reduce JavaScript payloadsave ~0.018 g/visit (129 kB)Medium impact · moderate effort

   6 scripts total 516 kB on the wire (740 kB decoded). Bundles this size usually carry a lot of unused code.

   Split bundles per route, tree-shake dependencies, and audit with a bundle analyzer or coverage profile. A 25% cut is a conservative target for bundles this size.

   • main.js376 kB
   • gtm.js123 kB
   • modernizr.js6.3 kB
   • SpotlightsRecent.d6a3aa4b97b1659f1bbb.js5.4 kB
   • Notification.11fc703b491d61aecf97.js3.8 kB

   WSG 3.2: Minify and remove unused codeWSG 3.3: Modularize bandwidth-heavy components
6. Audit third-party servicessave ~0.018 g/visit (129 kB)Medium impact · moderate effort

   3 third-party domains account for 257 kB (3% of the page).

   Remove services that no longer earn their place; load the rest on interaction (facades for embeds, delayed consent-gated tags). Each third party is also a privacy and performance dependency.

   • use.typekit.net134 kB
   • www.googletagmanager.com123 kB
   • p.typekit.net172 B

   WSG 3.5: Treat third parties the same as first parties
7. Serve images at the size they displaysave ~0.0076 g/visit (53 kB)Medium impact · quick fix

   2 images are encoded at more than twice the pixels the layout shows, even accounting for this viewport's pixel density.

   Generate multiple sizes and let the browser pick with srcset/sizes; a 4000px original behind a 400px slot spends bandwidth no screen displays.

   • MIT-aerial-3-EmilyDahl_0d.jpg50 kB
   • MIT-Hugh-Smith-01-thumb.jpg46 kB

   WSG 2.9: Optimize media to reduce resource use
8. Verify your hosting energyMedium impact · moderate effort

   This host has not published renewable energy evidence with the Green Web Foundation, so its energy source is unknown.

   Choose a provider listed in the Green Web Directory, or ask your current provider to publish evidence (and a carbon.txt). This cuts the data-center share of every single page view.

   • carbon.txtNo carbon.txt found at the site root; publishing one is the lightest first step.

   WSG 4.1: Use sustainable hosting
9. Optimize web fontssave ~0.0066 g/visit (46 kB)Advisory · quick fix

   5 font files total 133 kB.

   Subset fonts to used glyphs, prefer one variable font over multiple weights, serve WOFF2 only, and consider system fonts for UI text.

   • l29 kB
   • l28 kB
   • l26 kB
   • l26 kB
   • l23 kB

   WSG 2.11: Use optimized web typography
10. Set a page-weight budgetAdvisory · moderate effort

    At 8.74 MB per view this page rates F. The findings above list where the weight is.

    Agree a transfer budget (e.g. under 975 kB reaches a B), enforce it in CI, and re-scan after each change to hold the line.

    WSG 3.1: Set goals based on performance and energy impact
11. Remove the entry redirectAdvisory · quick fix

    The address that was scanned goes through 1 redirect before the document is served, ending at https://web.mit.edu/. Every hop is a full round trip on a connection that has not warmed up yet.

    Point links, sitemaps and canonical tags at the final address, and make the host redirect in one hop rather than chaining http to https to www.

    • https://mit.edu/redirects to https://web.mit.edu/

    WSG 4.4: Setup necessary error pages and redirection links
12. Upgrade connections to HTTP/2 or HTTP/3Advisory · quick fix

    1 host serves five or more resources over HTTP/1.1, paying per-request connection and header overhead newer protocols eliminate.

    Enable HTTP/2 (or HTTP/3) on the server or move the assets behind a CDN that speaks it; multiplexing makes many small requests nearly free.

    • web.mit.edu17 requests over http/1.1

    WSG 3.1: Set goals based on performance and energy impact

Where the bytes are

• Images8.00 MB · 7 req
• JavaScript516 kB · 6 req
• Fonts133 kB · 5 req
• CSS79 kB · 3 req
• HTML12 kB · 1 req
• Data (XHR)3.7 kB · 4 req