Advanced GTM: Dynamic Measurement IDs for Multiple Live Sites

The Challenge: One GTM Container, Multiple Live Sites

You’re managing multiple live websites or domains using the same GTM container:

Main company site (yourcompany.com)
E-commerce store (shop.yourcompany.com)
Blog or content site (blog.yourcompany.com)
Multiple client sites (for agencies)
Different product lines or brands

The problem: Each site needs its own GA4 property for proper reporting, but managing separate GTM containers creates maintenance nightmares and inconsistent implementations.

The solution: Dynamic measurement ID switching based on the current domain, all within one GTM container.

Why This Advanced Setup Matters

For businesses with multiple properties:

Clean data separation between different sites/brands
Unified GTM management (one container to maintain)
Consistent tracking implementation across all properties
Simplified tag and trigger management
Easy addition of new sites without duplicating work

Real-world scenarios:

Agency managing multiple client websites
Company with separate e-commerce and content sites
Multi-brand organizations with distinct properties
SaaS companies with separate product sites
Franchise businesses with location-specific sites

Prerequisites

This tutorial builds on our previous posts in this GTM series:

You’ll need:

Multiple live websites or domains to track
Separate GA4 properties set up (one for each site)
Understanding of GTM variables and lookup tables
Admin access to your GTM container

The Complete Setup

Step 1: Create GA4 Properties for Each Site

In Google Analytics:

Create separate GA4 properties for each site:

Main Site: G-MAIN123456 (yourcompany.com)
E-commerce: G-SHOP789012 (shop.yourcompany.com)
Blog: G-BLOG345678 (blog.yourcompany.com)
Client A: G-CLIENTA901234 (clienta.com)

Pro tip: Use descriptive property names like “YourCompany – Main Site” and “YourCompany – E-commerce” to avoid confusion in reporting.

Step 2: Create Your Domain-to-Site Mapping Variable

Create a new GTM Variable:

Variable name: Regex Table - Site Mapping
Variable type: Regex Table
Input Variable: {{Page Hostname}}

Pattern/Output Configuration:

Pattern: yourcompany.com
Output: main_site

Pattern: shop.yourcompany.com
Output: ecommerce

Pattern: blog.yourcompany.com
Output: blog

Pattern: clienta.com
Output: client_a

Pattern: clientb.com
Output: client_b

Default Value: unknown_site

Advanced Settings:

✅ Ignore Case (checked)
❌ Full Matches Only (unchecked)
❌ Enable Capture Groups (unchecked)

Why this approach: Using descriptive site identifiers makes the lookup table more readable and maintainable than using measurement IDs directly.

Step 3: Create the Dynamic Measurement ID Variable

Variable name: GA4 - Measurement ID - Dynamic
Variable type: Lookup Table
Input Variable: {{Regex Table - Site Mapping}}

Input/Output Configuration:

Input: main_site
Output: G-MAIN123456

Input: ecommerce
Output: G-SHOP789012

Input: blog
Output: G-BLOG345678

Input: client_a
Output: G-CLIENTA901234

Input: client_b
Output: G-CLIENTB567890

Default Value: G-DEFAULT999999

Why use a default: If a new site is added that doesn’t match any patterns, it uses a designated “catch-all” GA4 property rather than breaking tracking.

Step 4: Update Your GA4 Configuration Tag

Edit your main GA4 Configuration tag:

Tag Type: Google Analytics: GA4 Configuration
Measurement ID: {{GA4 - Measurement ID - Dynamic}}
Trigger: Initialization – All Pages

Remove any hard-coded measurement IDs from this tag. The dynamic variable now handles environment detection automatically.

Step 5: Add Site-Specific Custom Dimensions

Add site identification to all properties:

In your GA4 Configuration tag, add:

Parameter: site_identifier
Value: {{Regex Table - Site Mapping}}

This creates a custom dimension showing which site each hit came from, useful for:

Cross-site reporting in GA4
Debugging tracking issues
Understanding traffic patterns across properties

Advanced Configuration Options

Option 1: Site-Specific Event Configuration

Different conversion values by site:

Variable name: GA4 - Default Currency - Site
Variable type: Lookup Table
Input Variable: {{Regex Table - Site Mapping}}

Configuration:

Input: main_site
Output: USD

Input: ecommerce
Output: USD

Input: client_a
Output: EUR

Input: client_b
Output: GBP

Option 2: Site-Specific Enhanced Measurement

Control enhanced measurement per site:

Variable name: GA4 - Enhanced Measurement - Site
Variable type: Lookup Table
Input Variable: {{Regex Table - Site Mapping}}

Configuration:

Input: main_site
Output: true

Input: ecommerce
Output: true

Input: blog
Output: false

Input: client_a
Output: true

Add to GA4 Configuration:

Parameter: allow_enhanced_conversions
Value: {{GA4 - Enhanced Measurement - Site}}

Option 3: Site-Specific Custom Parameters

Add site-specific context:

Variable name: GA4 - Site Type - Classification
Variable type: Lookup Table
Input Variable: {{Regex Table - Site Mapping}}

Configuration:

Input: main_site
Output: corporate

Input: ecommerce
Output: ecommerce

Input: blog
Output: content

Input: client_a
Output: client_site

Testing Your Dynamic Setup

Test 1: Main Company Site

In GTM Preview Mode:

Connect to your main site (yourcompany.com)
Click Initialization event
Verify in GA4 Configuration tag:
- Measurement ID shows G-MAIN123456
- Site identifier shows main_site
- Currency shows USD (if configured)

Test 2: E-commerce Site

In GTM Preview Mode:

Connect to your e-commerce site (shop.yourcompany.com)
Click Initialization event
Verify in GA4 Configuration tag:
- Measurement ID shows G-SHOP789012
- Site identifier shows ecommerce
- Enhanced measurement enabled

Test 3: Client Site

In GTM Preview Mode:

Connect to a client site (clienta.com)
Click Initialization event
Verify in GA4 Configuration tag:
- Measurement ID shows G-CLIENTA901234
- Site identifier shows client_a
- Correct currency and site type

Test 4: Real-Time Verification

Check that data reaches the correct GA4 properties:

Go to each GA4 property’s Real-time report
Navigate to the corresponding site
Verify traffic appears in the correct property only
Check that custom dimensions populate correctly

Enterprise-Level Enhancements

Multi-Brand Company Setup

For companies with multiple brands:

Variable name: Brand - Domain Mapping
Variable type: Regex Table
Input Variable: {{Page Hostname}}

Pattern examples:

Pattern: brandone.com
Output: brand_one

Pattern: brandtwo.com  
Output: brand_two

Pattern: shop.brandone.com
Output: brand_one_shop

Then create measurement ID lookup:

Input: brand_one
Output: G-BRAND1MAIN

Input: brand_one_shop
Output: G-BRAND1SHOP

Input: brand_two
Output: G-BRAND2MAIN

Franchise or Multi-Location Setup

For franchise businesses or multi-location companies:

Pattern: denver.yourfranchise.com
Output: location_denver

Pattern: chicago.yourfranchise.com
Output: location_chicago

Pattern: newyork.yourfranchise.com
Output: location_newyork

Each location gets its own GA4 property while maintaining consistent GTM implementation.

Dynamic Marketing Pixel Configuration

Use the same site mapping for marketing pixels:

Variable name: Facebook Pixel ID - Site
Variable type: Lookup Table
Input Variable: {{Regex Table - Site Mapping}}

Input: main_site
Output: 1234567890

Input: ecommerce
Output: 0987654321

Input: client_a
Output: 1122334455

This ensures each site uses the correct Facebook Pixel, LinkedIn tag, etc.

Troubleshooting Common Issues

Wrong measurement ID being used:

Check that domain patterns are specific enough and don’t overlap
Verify the lookup table has all sites covered
Test the site mapping variable in GTM preview mode

Data not appearing in expected GA4 property:

Confirm the measurement IDs are correct in the lookup table
Check that the GA4 Configuration tag is firing on Initialization
Verify network requests in browser dev tools show the correct measurement ID

Cross-site data contamination:

Ensure domain patterns are specific and don’t match multiple sites
Check for hardcoded measurement IDs in other tags
Verify that each site’s data appears only in its designated GA4 property

New sites not tracking correctly:

Add the new domain pattern to the site mapping variable
Create corresponding measurement ID in the lookup table
Test the new pattern matches correctly in preview mode

Performance Considerations

GTM container size:

This setup adds minimal overhead to your GTM container
Variables are evaluated once per page load
Lookup tables are more efficient than complex JavaScript

Cross-domain tracking:

If sites need to share user sessions, configure cross-domain tracking
Add linker parameters to the GA4 Configuration tag
Ensure consistent measurement IDs for related sites

Maintenance and scalability:

Adding new sites requires only variable updates
No need to duplicate tags, triggers, or complex configurations
Version control friendly (all logic in GTM workspace)

When to Use This Advanced Setup

Perfect for:

Agencies managing multiple client websites
Companies with separate brands or product lines
Multi-domain e-commerce operations
Franchise businesses with location-specific sites
Organizations with distinct content and commerce properties

Probably overkill for:

Single-site businesses
Sites that can share the same GA4 property
Teams without dedicated analytics expertise
Simple blog or brochure websites

Professional Implementation Services

This advanced setup demonstrates enterprise-level GTM capabilities, but complex implementations often benefit from professional guidance:

Common scenarios requiring expert help:

Multi-brand GTM governance and strategy
Complex e-commerce tracking across multiple sites
Cross-domain tracking and attribution setup
Advanced conversion tracking and audience building
Team training on GTM best practices and maintenance

What’s included in professional multi-site GTM consulting:

Complete multi-site tracking architecture design
Site-specific analytics strategy and implementation
Cross-site reporting and attribution setup
Documentation and team training
Ongoing optimization and governance

Managing multiple websites with complex GTM tracking needs? Contact Knihter for expert multi-site GTM consulting and implementation services. We specialize in scalable tracking architectures for agencies and multi-brand organizations.

Related Posts in This Series:

Related Services:

Multi-site GTM implementation and governance