Setting Up Cross-Domain Tracking in GA4

The Problem: Session Fragmentation

When a user moves from your marketing site (example.com) to your checkout domain (shop.example.com), GA4 starts a new session and attributes the visit to "referral" from example.com. This breaks your funnel data and inflates referral traffic from your own properties.

When You Need Cross-Domain Tracking

• Your checkout is on a different subdomain or domain
• You use a booking platform (Calendly, Acuity) that redirects back
• You have a separate app domain and a marketing site
• You use a payment processor that redirects through their domain

GA4 Configuration

In GA4, go to Admin → Data Streams → your web stream → Configure tag settings → Configure your domains.

Add all domains that should be treated as one property:

• example.com
• shop.example.com
• checkout.example.com

Click Save. GA4 will now share the _ga cookie parameter across these domains via URL decoration.

GTM Alternative

If you manage your tag via GTM, add cross-domain tracking in your Google Tag configuration:

• Find your Google Tag in GTM (the base GA4 tag)
• Add configuration setting: "linker domains"
• Value: shop.example.com, checkout.example.com

Testing Cross-Domain Tracking

• Enable GTM Preview or use Google's Tag Assistant
• Navigate from your main site to the cross-domain destination
• Check the URL — it should contain a ?_gl= parameter with a session identifier
• In GA4 DebugView, confirm the session continues without a new session start

Common Issues

Referral exclusion: Also add your domains to the referral exclusion list (Admin → Data Streams → Referral Exclusions) as a belt-and-suspenders approach.

iFrame embeds: Forms embedded in iFrames from another domain require additional JavaScript communication — contact the form provider for cross-domain support.

Link decoration blocked: Some browsers or security policies strip URL parameters. Server-side cross-domain tracking solves this but requires more setup.