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Marketing Data Engineer > Google Tag Manager > How to Set Up Cross-Domain Tracking with Google Tag Manager (GTM)
Google Tag Manager

How to Set Up Cross-Domain Tracking with Google Tag Manager (GTM)

zingermk@gmail.com 10 Min Read
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If you run a small business with multiple websites—or use a third-party booking or checkout platform you might be missing key insights in your Google Analytics 4 (GA4) reports.

Contents
What Is Cross-Domain Tracking And Do You Even Need It?When Do You Need to Set Up Cross-Domain Tracking?Step-by-Step: How to Set Up Cross-Domain Tracking in GTMStep 1: Add Domains to Your GA4 Configuration Tag in GTMStep 2: Add Domains in GA4 Admin SettingsStep 3: Enable Auto Link DomainsHow to Test if Cross-Domain Tracking is WorkingTroubleshooting Cross-Domain TrackingDoes GA4 Support Subdomain Tracking?Why It Matters for Your Business

 

Ever notice your traffic numbers don’t add up? Or that visitors suddenly become “new users” halfway through their journey?

That’s where cross-domain tracking comes in.

 

And don’t worry it sounds more technical than it really is. I’ll guide you through the process using Google Tag Manager (GTM), real-life examples, and step-by-step instructions you can follow today.

What Is Cross-Domain Tracking And Do You Even Need It?

Cross-domain tracking is a powerful method for monitoring user interactions across multiple websites or domains. For small businesses with more than one web property whether it’s separate sites for different products, services, or even different marketing campaigns cross-domain tracking ensures that all user behavior is tracked cohesively.

 

Without it, Google Analytics will treat visits to different domains as separate users, leading to fragmented data and missed insights. By implementing cross-domain tracking, you can capture a unified view of the customer journey, even as users navigate between different websites. This becomes essential for businesses that want to fully understand their customers’ interactions across different platforms.

 

The benefits of cross-domain tracking for small businesses include:

 

  • Unified Customer Journey: Track users across multiple domains and see how they interact with different parts of your business, offering a clearer picture of their behavior.

  • Improved Conversion Tracking: Measure conversions more accurately by understanding how users move between domains, which is crucial for optimizing the sales funnel.

  • Better Data Accuracy: Prevent data fragmentation and ensure that user sessions are not split across multiple reports, leading to more reliable insights.

  • Enhanced Marketing Insights: Understand how different campaigns, websites, and services perform together, allowing you to optimize marketing efforts.

  • Optimized Customer Experience: With a better understanding of user behavior, you can improve site navigation and design to enhance user experience and increase conversions.

When Do You Need to Set Up Cross-Domain Tracking?

Cross-domain tracking is necessary when your users navigate between two or more completely different domains, and you want to track that journey as a single session in GA4. For example, if your main website is yourbusiness.com but your checkout happens on yourstore.com, you’ll need to configure cross-domain tracking so GA4 doesn’t break that session into two separate users.

You should consider installing cross-domain tracking if:

 

  • 🔗 You use a separate domain for your online store, checkout, booking engine, or payment portal (e.g., from yourbusiness.com to yourstore.com).

  • 🎯 Your lead capture forms or landing pages are hosted on a different domain or marketing platform (e.g., Unbounce, Leadpages).

  • 🌐 You manage multiple branded websites under the same business and want to track user flow between them.

  • 🧾 You use third-party platforms (like event registration or client portals) that sit on their own domains but are part of your customer journey.

  • 📈 You want to create accurate funnel reports or attribution models in GA4 that include user actions across multiple domains.

 

In these cases, cross-domain tracking helps you see the full picture of how your visitors interact with your business — from the first touchpoint to conversion — no matter which domain they visit along the way.

Step-by-Step: How to Set Up Cross-Domain Tracking in GTM

Step 1: Add Domains to Your GA4 Configuration Tag in GTM

  1. Open Google Tag Manager

  2. Go to Tags → GA4 Configuration

  3. Under Fields to Set, add:

    • Field Name: allowLinker → Value: true
    • Field Name: linker → Value: { 'domains': ['domain1.com', 'domain2.com'] }

    Replace domain1.com and domain2.com with your actual domains.

Step 2: Add Domains in GA4 Admin Settings

    1. Go to GA4 → Admin → Data Streams
    2. Click your data stream
    3. Scroll to Tagging Settings → Click Configure your domains
    4. Add all domains you want to track, like:

Step 3: Enable Auto Link Domains

If you didn’t do it manually in GTM, GA4 can try to auto-decorate links:

  1. In GA4 → Admin → Data Stream
  2. Under Tagging Settings, enable Cross-domain auto linking

Manual setup is more reliable, especially if you have embedded forms or iframes.

How to Test if Cross-Domain Tracking is Working

Now that you know why cross-domain tracking is important, let’s talk about how to check if it’s set up correctly. Testing is key to making sure your data is being tracked as expected.

Here’s a simple way to test:

 

  1. Open Your Website in a New Incognito Window: This ensures you’re starting with a fresh session.

  2. Navigate Between Domains: Go from one domain to another (for example, from your homepage to your checkout page).

  3. Check Google Analytics Real-Time Report: While on your second domain, open your Google Analytics real-time report and confirm that the session is carried over without showing up as a new user.

  4. Use the Tag Assistant Tool: Google’s Tag Assistant Chrome extension can help you check if your tags are firing correctly across domains.

 

If everything is working, you should see the user session tracked across the domains without being reset.

Troubleshooting Cross-Domain Tracking

Even if you follow all the steps correctly, issues can sometimes arise. Here’s how to troubleshoot common problems with cross-domain tracking:

 

  1. Sessions Resetting Between Domains: If you’re seeing users as separate sessions across domains, it could mean your cross-domain tracking is not set up correctly. Double-check your Google Tag Manager or tracking code to make sure the correct domains are listed.

  2. Missing Referral Information: If you’re losing referral data between domains, ensure that the “allow linker” parameter is properly configured in your tag setup. This helps pass session information between domains.

  3. Cookie Settings: Cross-domain tracking relies on cookies to pass session data. If your cookie settings are too restrictive (like using third-party cookies), it can break the tracking. Make sure cookies are set to work across domains.

  4. Tag Firing Issues: Use the Tag Assistant Chrome extension to check if your tags are firing as expected. If tags are missing or firing incorrectly, that could be a reason data isn’t being tracked across domains.

Does GA4 Support Subdomain Tracking?

Yes , Google Analytics 4 (GA4) automatically supports subdomain tracking right out of the box. That means if your business runs multiple subdomains (like shop.yoursite.com, blog.yoursite.com, or support.yoursite.com), GA4 will treat visits across those subdomains as part of the same session, as long as your GA4 tag is implemented correctly on each one.

 

You don’t need to do anything extra to enable subdomain tracking in GA4. Just make sure the same GA4 measurement ID is used on all subdomains, and you’ll be able to see a unified view of user behavior across them.

Why It Matters for Your Business

Cross-domain tracking is an essential tool for your small businesses that manage multiple websites or domains. By setting it up correctly, you can get a clear, unified view of your customer journey, leading to better marketing insights, improved customer experiences, and more accurate data for decision-making.

 

If you’ve been relying on fragmented data, now’s the time to implement cross-domain tracking and start seeing the bigger picture. While it may require a little technical setup, the benefits are well worth the effort. Plus, once it’s set up, you’ll have the ability to track every step of your customers’ journey across your digital ecosystem.

 

Ready to get started? Test your current setup, troubleshoot any issues, and keep your data flowing seamlessly across domains to optimize your marketing efforts.

zingermk@gmail.com 05/16/2025
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