GA4 User ID and Cross-Device Tracking

GA4 User ID and Cross-Device Tracking

Complete guide to User ID implementation, user properties, and cross-device tracking in GA4.

Overview

User ID enables cross-device tracking for authenticated users. User properties allow tracking custom user attributes across all events. Together, they provide a unified view of user behaviour across devices and sessions.

User ID Implementation

What is User ID

• Persistent identifier for authenticated users
• Enables cross-device tracking
• Requires explicit user authentication
• Must not contain PII (hash if needed)
• Set when user logs in, cleared on logout

Implementation Methods

Method 1: gtag.js

On Login:

// Set User ID when user logs in
gtag('config', 'G-XXXXXXXXXX', {
  'user_id': 'USER_12345'
});

// Or use gtag('set')
gtag('set', 'user_id', 'USER_12345');

On Logout:

// Clear User ID - MUST use null, never empty string
gtag('set', 'user_id', null);

Method 2: GTM Data Layer

Push to Data Layer:

// On login
dataLayer.push({
  'user_id': 'USER_12345'
});

// On logout
dataLayer.push({
  'user_id': null
});

GTM Configuration:

1. Create Data Layer Variable: user_id
2. In GA4 Configuration tag, add parameter:
   • Parameter: user_id
   • Value: {{DL - User ID}}

Method 3: Measurement Protocol

{
  "client_id": "client_123",
  "user_id": "USER_12345",
  "events": [
    {
      "name": "login",
      "params": {
        "method": "email"
      }
    }
  ]
}

User ID Best Practices

User ID Format Examples

Good:

• USER_12345 (internal ID)
• abc123def456 (hashed value)
• u_a1b2c3d4 (prefixed ID)

Bad:

• john@example.com (email - PII)
• +1-555-123-4567 (phone - PII)
• John Smith (name - PII)

Reporting Identity

Configure how GA4 identifies users across sessions.

Path

Admin -> Data Settings -> Reporting Identity

Options

Blended (Recommended)

• Uses User ID when available
• Falls back to Device ID + Google Signals
• Most complete user view
• Enables cross-device insights

Observed

• Uses User ID and Device ID only
• No Google Signals data
• More privacy-focused
• Fewer users identified

Device-based

• Uses only client_id (device)
• No cross-device tracking
• Maximum privacy
• Limited user insights

User Properties

What are User Properties

• Custom attributes set at user level
• Persist across all events from that user
• Different from event parameters
• Must be registered as custom dimensions to report

Limits

Setting User Properties

gtag.js:

gtag('set', 'user_properties', {
  'user_tier': 'premium',
  'account_age_days': 365,
  'preferred_category': 'electronics'
});

GTM Data Layer:

dataLayer.push({
  'user_properties': {
    'user_tier': 'premium',
    'account_age_days': 365
  }
});

Measurement Protocol:

{
  "client_id": "client_123",
  "user_properties": {
    "user_tier": {
      "value": "premium"
    },
    "account_age_days": {
      "value": 365
    }
  },
  "events": [...]
}

Common User Properties

PII Considerations

Avoid:

• email
• name
• phone_number
• address
• credit_card

Use Instead:

• email_domain
• first_name_initial
• user_segment
• location_region

Cross-Device Tracking

How It Works

1. User visits on mobile (logged in) -> user_id set
2. User visits on desktop (logged in) -> same user_id
3. GA4 stitches sessions together
4. Reports show unified user journey

Requirements

• User ID implemented and set on login
• Google Signals enabled (for Blended identity)
• Users signed into Google accounts
• User opt-in to personalisation

Enabling Google Signals

1. Admin -> Data Settings -> Data Collection
2. Enable Google Signals
3. Accept terms
4. Wait 24 hours for data

Cross-Domain Tracking

For User ID across domains:

gtag.js Configuration:

gtag('config', 'G-XXXXXXXXXX', {
  'linker': {
    'domains': ['example.com', 'shop.example.com', 'blog.example.com']
  }
});

What This Does:

• Passes client_id between domains via URL parameter
• Preserves user_id if set
• Maintains session continuity
• Uses _gl URL parameter

GTM Setup:

1. In GA4 Configuration tag
2. Expand Configuration Settings
3. Add Fields to Set:
   • Field Name: linker
   • Value: {"domains": ["example.com", "shop.example.com"]}

Testing Cross-Domain

1. Visit domain1.com
2. Click link to domain2.com
3. Verify _gl parameter in URL
4. Check DebugView shows same user
5. Confirm session continues

User-Scoped Custom Dimensions

Register user properties for reporting:

Registration

1. Admin -> Data Display -> Custom Definitions
2. Click "Create custom dimension"
3. Configure:
   • Dimension name: "User Tier"
   • Scope: User
   • User property: user_tier
4. Save

Using in Reports

• Add as dimension in Explorations
• Filter/segment by dimension
• Analyse user cohorts
• Compare user segments

User Data Deletion

GDPR/CCPA Compliance

Path: Admin -> Data Settings -> Data Deletion Requests

Process

1. Click "Create deletion request"
2. Select parameter: User ID
3. Enter User ID value to delete
4. Choose scope: All data or date range
5. Submit request

Processing

• Takes up to 72 hours
• Deletes ALL events for that User ID
• Cannot be undone
• Confirmation email sent

Use Cases

• GDPR right to erasure requests
• CCPA deletion requests
• User account deletion
• Data cleanup

Testing User ID

Verification Workflow

1. Implement User ID
2. Enable DebugView
3. Test Login:
   • Before login: Events have only client_id
   • After login: Verify user_id in event details
4. Test Logout:
   • After logout: Verify user_id cleared/null
5. Test Cross-Device:
   • Login on Device 1
   • Login on Device 2 with same account
   • Verify same user_id

DebugView Verification

1. Open Admin -> DebugView
2. Click any event
3. Expand event details
4. Check "User ID" field populated

Reports Verification

• Wait 24-48 hours
• Check user count vs sessions
• Lower user count = User ID working (same user, multiple sessions)

Common Issues

User ID Not Appearing

Causes:

• Set after gtag config
• Cleared immediately
• Wrong parameter name

Solutions:

1. Set user_id in config call
2. Verify login flow
3. Check parameter is 'user_id' exactly

Cross-Device Not Working

Causes:

• Different User IDs
• User ID not set on all devices
• Reporting Identity not configured

Solutions:

1. Verify same User ID across devices
2. Ensure login sets User ID
3. Check Reporting Identity = Blended

User Properties Not in Reports

Cause: Not registered as custom dimensions

Solution:

1. Register as user-scoped custom dimension
2. Wait 24-48 hours
3. Use in Explorations

Quick Reference

Set User ID

gtag('config', 'G-XXXXXXXXXX', {
  'user_id': 'USER_123'
});

Set User Properties

gtag('set', 'user_properties', {
  'user_tier': 'premium'
});

Clear User ID

gtag('set', 'user_id', null);

Limits

• 25 user properties per property
• User property name: 24 characters max
• User property value: 36 characters max
• User ID: Must not contain PII