GA4 Cross-Domain Setup Steps | How to Measure Across Multiple Sites

It is not unusual for a single service to span multiple domains, such as an EC site and a payment page, or a corporate site and a landing page on a separate domain. In GA4, cross-domain configuration is needed to correctly connect and measure user behavior that crosses these domains. If you neglect this configuration, the session splits the moment a user moves between domains, and you can no longer accurately trace conversion paths. This article explains the steps for GA4 cross-domain configuration concretely, following the admin screen operations.

What is cross-domain configuration

Cross-domain configuration is a setting that measures a user who moves between different domains as the same user in the same session. Because GA4 identifies users with first-party cookies issued per domain, without this configuration a user who moves from domain A to domain B is treated as a different person.

When you enable cross-domain configuration, a parameter called "_gl" is automatically appended to the URL when a link to a target domain is clicked, and the user ID is carried over through this parameter. This lets you measure behavior across multiple sites as a single continuous flow.

Prerequisites to confirm before configuring

Confirming the following points before starting the work will make it go smoothly.

• The same GA4 tag with the same measurement ID (an ID starting with G-) is installed on all domains you want to measure
• You have edit permission for the target property
• You have listed out all the domains you want to measure across
• Subdomains sharing the same root domain do not need cross-domain configuration as long as they have the same tag installed

Especially important is that all domains use the same measurement ID. If the measurement IDs differ, cross-domain measurement will not work at all.

Steps for cross-domain configuration in the GA4 admin screen

In GA4, the configuration is completed entirely within the admin screen, without using Google Tag Manager. The steps are as follows.

1. Log in to GA4, open the target property, and click "Admin" at the bottom left of the screen
2. Select "Data Streams" and click the web stream you want to configure
3. On the web stream details screen, click "Configure tag settings"
4. Open "Configure your domains" from the settings menu
5. Click "Add condition," select a match type (such as "contains"), and enter the target domain
6. Add all the domains you want to measure across, then click "Save"

In GA4, registering the other party's domain on just one of the sites enables cross-domain measurement in both directions. For example, if you register site B in site A's property, transitions from B to A are also measured as the same user.

Do not forget to register in the "unwanted referrals" list

Along with the cross-domain configuration, add all the domains you want to measure to "Data Streams" → "Configure tag settings" → "List unwanted referrals." If you neglect this, transitions between domains are recorded as referral traffic, causing sessions to split.

How to confirm the configuration is working correctly

After configuring, always confirm that cross-domain measurement is actually working. There are three main confirmation methods.

• Click a link from domain A to domain B and confirm that the "_gl=" parameter is appended in the address bar of the destination URL
• In GA4's realtime report, confirm that operations crossing domains are recorded as a single session
• Use DebugView to check each event's parameters and verify that the client ID matches across both domains

How to view data by domain

When you enable cross-domain configuration, URLs are displayed on reports aggregated at the path level (the top page shows as "/"). If you want to check by domain, open "Reports" → "Engagement" → "Pages and screens" and add "Hostname" as a secondary dimension. This lets you distinguish which domain the access went to and analyze accordingly.

Checkpoints when measurement is not working

If sessions do not connect even though you configured everything, check the following points.

• Whether there are typos in the registered domain names, and whether the protocol (http vs https) is specified correctly
• Whether there is a mix of www and non-www
• Whether the _gl parameter is lost mid-transition on pages that redirect
• Whether the parameter is appended on transitions via JavaScript-generated links or form submissions
• If you embed external domains via iframe, whether additional configuration has become necessary

Especially common are a mix of www and non-www, and the loss of the _gl parameter due to redirects. Because these are hard to confirm visually, inspecting network requests with Chrome developer tools as needed makes it easier to identify the cause.

Summary

GA4 cross-domain configuration is an important setting for accurately measuring user behavior that crosses multiple sites. The basic flow is to install the same measurement ID on all domains, register the target domains in "Configure your domains" in the admin screen, and add the same domains to the "list unwanted referrals." After configuring, confirm the _gl parameter and session continuity with DebugView and the realtime report, and verify that measurement works correctly. Accurate cross-domain measurement greatly improves the accuracy of analyzing conversion paths across domains.