Measure & Improve Your Site's Footprint with Carbon Control from Catchpoint WebPageTest

Today we're proud to announce the release of Carbon Control, an exciting new feature in Catchpoint WebPageTest.

Central to this new feature is the inclusion of a new experimental metric in WebPageTest for measuring a website's Carbon Footprint. Measuring carbon footprint has been the most requested and discussed issue in our tracker, and for good reasons: the web community is increasingly concerned about reducing our contributions to climate change, and companies will increasingly be held accountable for monitoring and reducing their emissions. At Catchpoint WebPageTest, we find this challenge worthy and interesting and we're proud to contribute tools to the effort.

But Carbon Control goes farther than merely measuring your footprint... it allows you to take action! That's where the "control" comes in. So let's take a look at what's included.

anchorIntroducing Carbon Control

Screenshot of carbon control start page

As of today, tests on will offer a carbon footprint measurement. On the homepage, you'll need to check the box to opt into running the new metric, or you'd prefer, you can use this handy Carbon Control start page to opt-in and go straight to the Carbon Control result page when the test is complete.

screenshot of new metric summary

Try a Carbon Control Test Now

The new Carbon Footprint metric is calculated with the help of excellent tools from the Green Web Foundation: Green Check and CO2.js. For a given site, the new metric represents a carbon emission equivalent based on a site's page weight and how much of that weight comes from Green hosting providers (note that first and third-party hosts are factored into this calculation). You'll find this new metric listed in the test summary page alongside the other standard metrics WebPageTest measures, as well as on the new Carbon Control result page which can be accessed via the test results navigation.

Exploring Your Impact

The new Carbon Control result page has a number for sections worth highlighting. To start, you'll find information about the green hosting status of the various services that a site uses to fetch its resources, and the estimated carbon footprint metric. Just after that, we've included some contextual information about how a site's footprint compares to everyday situations like driving a car, how that footprint compares to top sites, and also a reminder that test conditions such as device and location can cause a site's footprint to change (particularly when third party advertising changes by region), so it's worth testing different locations to see!

screenshot of footprint in context section of results

Further down the page, you'll find a breakdown of which types of resources are contributing most to a site's weight:

screenshot of breakdown pie chart for resources that are contributing most to page weight

And a handy diagnostic check of how a site is handling several footprint-relevant practices...

screenshot of footprint in context section of results

...some of which pair with links to No-Code Experiments! Which brings me to perhaps the most exciting part of the tool:

anchorDon't Just Measure... Control & Improve!

Much like other observations that WebPageTest surfaces, Carbon Control is designed to tie into WebPageTest's No-Code Experiments, which allow you to test whether specific optimizations will reduce a site's carbon footprint by applying them to a live website!

screenshot of new metric summary

Knowing Which Experiments to Run...

Some experiments are likelier to impact footprint more than others, particularly those that reduce pageweight or block or move resources that are hosted on servers that are not deemed "green". For example, if a site is particularly image-heavy, it's worth experimenting with whether some of those images could be lazy-loaded so they are not requested until they're needed. You might also try experimenting with changing the way images are referenced by using Responsive Images markup, or referencing different versions of the images that are better-optimized for delivery.

Or perhaps a site is using a large amount of JavaScript to generate its content after HTML is delivered... in that case you could try experiments such as "Mimic Pre-Rendered HTML" paired with "Disable Scripts," which changes the way a site is delivered so that its final HTML is delivered up-front, and thus no longer needs to load JavaScript to generate that HTML either (saving time AND pageweight!).

Here's an example Experiment that did just that and reduced a site's Carbon Footprint by 0.92g per fresh page visit (circled in red)!

screenshot of experiment results showing a large reduction in carbon emissions due to moving clientside rendering to the server

You can also experiment with the potential improvement of moving specific files to green hosts (this is mimicked by our experiment proxy), and get an idea of just how much that might reduce your footprint as well!

Accessing No-Code Experiments

While some No-Code Experiments are included for free WebPageTest accounts, most of the powerful experiments we offer are part of WebPageTest Pro, our premium product. You can find out more and sign up for WebPageTest Pro here!

anchorShow Your Peers That You Care

As part of this initiative, we've created a badge you can display on your website or LinkedIn to spread the word about your commitment to measure and reduce your impact. You can find it here or at the bottom of the Carbon Control page:

The WebPageTest Carbon Control Badge

Or grab this snippet to paste onto your site:

<a href=""><img src="" alt="We Monitor Our Footprint - WebPageTest Carbon Control from Catchpoint"></a>


This is our first release of our new Carbon Control feature, and we expect to be making refinements in the coming weeks to tighten things up wherever needed. We've also been careful to note that while we're excited about this new metric, it is considered experimental and we'll be monitoring its results and reliability in the coming year. We also intend to link more experiments from Carbon Control as we find relevant areas to do so, so keep an eye out for that!

As always, if you run into any troubles or have questions or ideas, we'd love to hear from you in the issue tracker. Thanks!

A Footnote on Limitations

Currently, the Carbon Footprint metric is limited to tests that use Chromium browsers such as Chrome and Edge, and available only on the website (API tests are not yet supported for Carbon Control). Also, Carbon Control will not collect data on green hosting for sites that have a Content Security Policy that prevents the test from running. Those sites will see a Carbon Footprint value that is still useful but not quite as as fine-tuned as it could otherwise be.

The good news is that we expect an upcoming release to remove both of these limitations.

Scott Jehl is a Senior Experience Engineer at Catchpoint who cares about creating fast, compelling digital experiences that can be delivered to the broadest possible audience. He is the creator of the Lightning-Fast Web Performance Course, author of Responsible Responsive Design (A Book Apart, 2014), and co-author of Designing with Progressive Enhancement (New Riders, 2010). He is also a frequent speaker at web conferences around the world. More at

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