Measure activity across multiple domains

This article is for developers who need to measure multiple domains with the same Google tag. This article focuses on a gtag.js code implementation. You can also configure additional domains in your Google tag settings of your GA4 property.

Cross-domain measurement enables two or more related sites on separate domains to be measured as one. Google tags provide a common way for supported products to measure activity when your customer journeys span across multiple domains. Cross-domain measurement works for all Google products, including Google Analytics, Google Ads conversion measurement, and Floodlight conversions.

When to implement cross-domain measurement

Implement cross-domain measurement when you need to combine data for two or more domains.

Suppose you have a website at example-petstore.com, and your website's ecommerce component is hosted by a third-party shopping cart on another domain: example-commerce-host.com/example-petstore.

Without cross-domain measurement, a user who arrives to your online store and then proceeds to your third-party shopping cart is counted as two separate users, with two separate sessions of different durations. With cross-domain measurement, activity is captured as a single user.

How it works

The domain linker shares first-party measurement cookies between a source domain and a destination domain. The first-party measurement cookies are stored in a web browser, and can only be accessed by pages on the same domain.

Setup for cross-domain measurement is a two-step process:

1. The source domain decorates URLs that point to the destination domain so that they contain the first-party measurement cookie values of the source domain.
2. The destination domain checks for the presence of linked measurement cookies.

Cross-domain measurement with Google tags will add a linker parameter to URLs that point to the destination domain. The linker parameter is identified in URL query parameters with the key _gl:

https://www.example.com/?_gl=1~abcde5~

On the destination domain, tags are configured to check for linker parameters in the URL. If a valid linker parameter is found, the tag extracts the first-party measurement cookie and stores it.

Set up cross-domain linking

To set up cross-domain measurement on the source domain for URLs that point to the destination domain, you need to create a linker object. The linker will listen for clicks on links that point to destination domains, and it will automatically add a linker parameter to the URLs of those links.

The linker object requires a domains parameter, which is an array of one or more domains. This code will append the linker parameter to links on a page that points to a target domain 'example.com':

<script async src="https://www.googletagmanager.com/gtag/js?id=TAG_ID"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('set', 'linker', {'domains': ['example.com']});
  gtag('js', new Date());

  gtag('config', 'TAG_ID');
</script>

gtag('set', 'linker', {
  'domains': ['example.com', 'example-b.com']
});

Linker parameter reference

accept_incoming

Once a user arrives at a page on the destination domain with a linker parameter in the URL, your tags need to be configured to parse that parameter.

If the destination domain has been configured to automatically link domains, it will accept linker parameters by default. No additional code is required on the destination domain.

If the destination domain is not configured to automatically link domains, you can instruct the destination page to look for linker parameters. Set the accept_incoming property to true.

gtag('set', 'linker', {
  'accept_incoming': true
});

decorate_forms

To measure form data that is sent between multiple domains, set the decorate_forms property to true.

gtag('set', 'linker', {
  'domains': ['example.com'],
  'decorate_forms': true
});

domains

An array of one or more domains to be linked.

gtag('set', 'linker', {
  'domains': [
    'example1.com',
    'example2.com',
    'subdomain1.example3.com',
    'subdomain2.example3.com'
  ]
});

url_position

Certain content applications require you to use a fragment/hash character (#) as the delimiter in URL strings instead of the more commonly used question mark character (?) to indicate query parameters. To configure the linker parameter to appear in the URL after a # character (e.g. https://example.com#_gl=1~abcde5~), set url_position to fragment.

gtag('set', 'linker', {
  'domains': ['example.com'],
  'decorate_forms': true,
  'url_position': 'fragment'
});