How does the Website Carbon calculator work?

Please see our ‘How does it work?‘ page for details.

How is v3 different to v2?

The v3 methodology was launched on Earth Day 2022 (22nd April) and uses newer data. The results produced report lower emissions for websites tested, reflecting that the infrastructure of the web and global power grids have become more efficient over time.

If you have tested your web pages prior to 22nd April 2022 and recorded a result and now want to test your improvements, testing using the v3 version will not be a like for like comparison. If you need to run a test using the older v2 methodology, this is possible using the API. Please ask  us for details.

Why does the test fail on my website?

We sometimes have trouble testing a url. This is generally for one of two reasons:

  1. If your website is very slow, the test may time out.
  2. If there are routing issues in your DNS settings, it can make it impossible for our test to load the page

Please check your load times and DNS settings before asking us to investigate a failed test.

I use green hosting, but your test says I don’t. Why?

Sometimes we cannot detect green hosting for the following reasons:

  1. If your hosting provider has not registered the IP address of your server with The Green Web Foundation, we will not be able to detect it. Please check with your hosting provider to see if this is the case.
  2. If you are using a CDN, then we will not be able to identify the IP address of your main web host and the test will judge the CDN provider as the host.
  3. Occasionally, we may not be able to obtain results for The Green Web Foundation API, in which case the test will state that the website appears to be using bog standard energy. When this happens, it is only ever temporary so try testing again the next day.

I changed my website but the test results don’t change. Why not?

We cache test results for up to a year to reduce energy consumption. If you are making changes to a website and testing each time, you won’t see any difference unless you click the “Test again” link at the top of your results page. This will then trigger a new test and you’ll get a new result straight away. This helps minimise the number of requests on the server, which is more energy efficient. We only allow retesting after 24 hours, so if you cannot see the button to test again, simply wait 24 hours and return to the page.

Please note that if you have the Website Carbon badge installed on your website, the badge is only cached for 7 days. As a result, the badge and the website may not always give the same result as they can be reporting tests conducting on different dates.

The Website Carbon badge on my website shows ‘No result’. What can I do?

If the Website Carbon badge on your website displays ‘No result,’ it is likely due to the Website Carbon API reaching its daily limit of requests when attempting to re-test your website. The badge is cached for 7 days, so it might take up to a week for the tests to run again. Please be patient, and the updated result should be reflected on your badge once the re-testing process is completed.

Can I test private urls?

It depends on how you define private. You can test pages that are blocked from search engines, so long as they can be accessed without a password. Please be cautious testing these as every test will generate a public, search engine indexable results page (see below about deleting such pages).

We cannot test pages protected by a password or on a local server. However, the carbon calculator code is open source so please do feel free to contribute a solution if this is something that you need.

Are test results public?

Yes, when you submit a url for testing, it generates a public report with the results.

Can you delete my test results?

If you have submitted a test and do not want the results public, then we have a simple policy.

  1. If you tested a private development url by accident (please don’t do it on purpose) and need to delete the public results page, please get in touch and we will delete the record as soon as we can.
  2. If you have tested a public url, we will not delete the results.

Can I add a badge to my website?

Yes, please find details on our badge page.

Can I edit the appearance of the badge?

Yes, you can override the CSS styles on the badge to fit more closely to your sites branding. You can find an editable version of the badge on Codepen.

Do you have an API?

We do have a website carbon API. Please check this page to find out more.

How do you calculate the average?

On the homepage of this website, we report the global average CO2 per page view of web pages. We used data from HTTP Archive. This is the median average and not the mean average. We have found the median to be more representative of a “typical” website as the mean average can be skewed by a small number of very polluting websites. For reference, the median is approximately 0.8 grams per page view as of July 2023.

What do you mean by CO2?

We use the term CO2 on this website to represent greenhouse gas emissions, often also referred to as carbon dioxide equivalent, or CO2e. For the purposes of this website, CO2 always means the same as CO2e. We use CO2 instead of CO2e in the main interfaces in order to make the content easier to understand for a general audience.

When might the score be wrong?

There are some websites that use sneaky optimisations to get around page load testing. There might also be genuine cases where our test isn’t able to detect the loading of some images or scripts. There are also cases where a website uses green hosting but we are unable to detect this. Unfortunately, as an automated test we can’t easily identify these without manually checking.

Where can I find more information about the Website Carbon Rating System?

We have summarised all key details about the Website Carbon Rating System on this page. The Website Carbon Rating System is based on the Digital Carbon Rating System developed through the Sustainable Web Design collaboration. For more in-depth details, you can visit this page.

How can I discover my Website Carbon rating?

To find your web page’s rating, test it on Website Carbon. Initially, you might see a cached result from before the introduction of our new rating system. Click the provided link on the cached results page to re-test your site, and you’ll see the updated Website Carbon rating for your web page.

Can I add the new rating to the Website Carbon badge on my website?

At the moment, the Website Carbon badge does not display the carbon rating. However, we are working on a new version of the badge that will include the rating. More details will be shared once the updated badge is available. If you’d like to stay informed about this and other developments, feel free to subscribe to our Curiously Green newsletter for the latest updates.

Can you conduct a more detailed sustainability audit?

Yes. Website Carbon is a public tool for general testing. If you would like a more in-depth digital sustainability analysis, please explore our Digital Sustainability services and get in touch with our team at Wholegrain Digital and we would be happy to discuss your requirements.

I have a question that isn’t answered here.

If you have any other questions or need help with something specific, send us an email at: [email protected]. Please note that we get a lot of emails and as this is a free tool, we are not able to reply to all enquiries.


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