This guide provides a strategic framework for implementing cross-domain measurement using Google Analytics 4 (GA4). Properly configuring cross-domain tracking allows you to understand user journeys across multiple related websites (e.g., your main website, a blog hosted on a separate domain, an e-commerce store, a campaign-specific landing page domain). This unified view is crucial for accurate attribution, holistic performance analysis, and optimized user experiences.
What is Cross-Domain Measurement?
Cross-domain measurement in GA4 allows you to track a single user's activity across multiple websites as a continuous session (with limitations explained below). Without it, a user navigating from domainA.com to domainB.com would be counted as two separate users with two separate sessions, making it impossible to understand the complete customer journey.
GA4 achieves cross-domain tracking by appending a linker parameter (_gl) to URLs that link between configured domains. This parameter shares the user's Client ID, allowing GA4 to recognize the same user on different sites.
Key Considerations Before You Begin
Before implementing cross-domain measurement, consider the following:
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Business Objectives: What are you trying to achieve with cross-domain tracking? Are you focused on unified user reporting, attribution modeling, or improving user experience? Your goals will inform your implementation strategy.
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Website Structure: How are your websites related? Do they share a common purpose, target audience, or conversion funnel?
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GA4 Account Structure: How is your current GA4 account set up? Do you use separate properties for each domain or a single property with multiple data streams?
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GA4 360 vs. Standard: Are you using the free (Standard) version of GA4 or the paid (360) version? GA4 360 offers features like Rollup Properties and Subproperties that can simplify cross-domain management but may not be necessary for all organizations. It also introduces considerations described below.
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Data Privacy and Compliance: Ensure your cross-domain implementation complies with all relevant data privacy regulations (e.g., GDPR, CCPA). Carefully consider data storage locations, user consent, and data governance.
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Technical Resources: Do you have access to the necessary technical experience in Google Tag Manager, GA4, and potentially BigQuery?
Choosing the Right Strategy
There are several ways to implement cross-domain tracking in GA4. Here are the four most common strategies:
- Single GA4 Property with a Single Data Stream - Recommended for most standard GA4 users; simple, unified setup with some caveats
- Multiple GA4 Properties with Cross-Domain Configuration - Best for BigQuery-centric analysis and strict data separation
- Rollup Properties (GA4 360 Only) – Best for high-level reporting across domains; limited session stitching
- Subproperties (GA4 360 Only) – Ideal for unified sessions and filtered views within a single property
Each strategy offers distinct advantages and trade-offs depending on your data structure, reporting needs and GA4 version. We’ve outlined them below—including how they work, when to use them, and the key pros and cons—to help you make an informed decision.
1. Single GA4 Property with a Single Data Stream
How it works:
You create a single GA4 property and configure a single data stream where all data is sent. GA4's built-in cross-domain configuration is configured within the data stream setting.
Pro Tip: To ensure unified sessions, add each respective website to the Referral Exclusion List to prevent traffic from the initial website from appearing as referral traffic.
Use Case:
Unified reporting without GA4 360. Recommended for most standard GA4 users, with caveats. This strategy offers a simple and effective setup for unified user journeys inside GA4.
Pros:
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Cons:
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- It is the most straightforward setup for unified user reporting.
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- Data Aggregation: All data is combined into a single data stream and single property, which might make it challenging to isolate data for individual domains (consider using Explorations or BigQuery for granular analysis). Also, any other settings applied at the data stream level will be applied across each website.
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- Consistent Client ID and Session ID across domains.
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- Data Governance: Managing user access and permissions can be challenging if different teams need access to data for specific domains only.
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- It is easiest to manage in the GA4 interface.
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- Data Limits: Standard GA4 has data limits (e.g., for BigQuery export) that could be exceeded if the combined traffic across all domains is very high.
- Historical Data: Data from previously separate properties stays separated.
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2. Multiple GA4 Properties with Cross-Domain Configuration
How it works:
Each domain has its own GA4 property with cross-domain tracking enabled in each data stream.
Use Case:
Best for strict data separation with external analysis. For when you need property-level separation but can join the data externally. Ideal for BigQuery-centric analysis.
Pros:
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Cons:
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- Better data separation and control.
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- Unified user reporting is not natively available in the GA4 interface. To join the data, you'll need to use BigQuery (recommended) or other tools based on the shared Client ID.
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- Easier to manage user access and permissions.
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- More complex setup and management.
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- Avoids data limits of a single property (for Standard GA4).
- Complies with any requirements for storing data separately.
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- Session Stitching Limitations: GA4's event-based data model means that sessions are not perfectly stitched together across source properties. You'll see the same user but with different session IDs.
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3. Rollup Properties (GA4 360 Only)
How it works:
A Rollup Property aggregates data from multiple source properties.
Use Case:
Best for aggregated reporting across properties. Ideal for viewing data from multiple domains together, with some session stitching limitations.
Pros:
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Cons:
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- Provides a unified view of user activity in the GA4 interface.
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- Session Stitching Limitations: GA4's event-based data model means that sessions are not perfectly stitched together across source properties, even in a Rollup Property. You'll see the same user but with different session IDs.
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- Simplifies reporting for organizations with many websites.
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- Requires a GA4 360 subscription.
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- Control access for subproperties and rollup properties, which can give you better data governance beyond just improved reporting capabilities.
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- Rollup property hits are billed at .5 hit on top of the initial hit within the source property.
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4. Subproperties (GA4 360 Only)
How it works:
Create subproperties that filter a subset of a source property.
Use Case:
Best for full cross-domain session continuity. Keep one Client ID and Session ID while offering filtered reporting views.
Pros:
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Cons:
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- It is possible to use the "Single GA4 Property with a Single Data Stream" approach to get shared Client IDs and Session IDs, but with the ability to split out the data.
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- Requires a GA4 360 subscription.
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- Allows for a single data collection point, but filtered views (such as by domain).
- Control access for subproperties and rollup properties, which can give you better data governance beyond just improved reporting capabilities.
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- Subproperty hits are billed at .5 hit on top of the initial hit within the source property.
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Important Technical Considerations
Now that you have a strategy in mind, it’s important to review a few key technical details to ensure your cross-domain measurement works as intended.
Multiple Properties
If using multiple properties, consider leaving domains visible in referral traffic to capture cross-domain attribution insights.
Link Decoration
GA4 uses the _gl parameter to share the Client ID across domains. This parameter is automatically added to links between configured domains. Make a note of these important considerations:
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Ensure that website redirects do not strip or modify the _gl parameter.
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Cross-domain tracking only works when users click on links between configured domains. Navigating directly to a different domain (e.g., typing the URL in the browser) will not maintain the cross-domain session.
Referral Exclusions
Sometimes, you may need to configure referral exclusions in GA4 to prevent your domains from appearing as referrals. Carefully consider the implications before adjusting referral exclusions. Using a Multiple Properties setup may be beneficial in not excluding referrals to see cross-domain clicks in GA4's attribution reports.
Enhanced Measurement
Enabling cross-domain measurement may disable automatic outbound link click tracking (part of GA4's Enhanced Measurement). You must configure custom events to track outbound clicks to your other domains.
User Consent
Implement a consistent consent management strategy across all domains to ensure compliance with privacy regulations.
Take-away Recommendation
For most organizations, using a single GA4 property with a single data stream is the best starting point for maintaining a unified view of users and sessions, especially if you are not using GA4 360.
If you have access to GA4 360, leveraging subproperties offers a more robust solution. It allows for unified user journeys across domains while providing filtered reporting views for different business units or sites within the same property.
Other approaches, such as Rollup Properties or Multiple Properties with cross-domain configuration, can still offer value depending on your reporting structure and analysis needs. However, they add extra complexity, especially if session unification is a top priority.
And if you need deeper analysis or greater flexibility across properties, BigQuery can enhance any of these strategies by enabling advanced custom reporting and data stitching capabilities.
Have additional questions? Need help getting started? Reach out to see how we can help!
