How do I set up user ID tracking in GA4?

Applies to: Google Analytics 4 (GA4)
Last updated: May 2025

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Problem

You want to consistently track logged-in users across devices and sessions using a unique identifier (e.g., customer ID or account email hash) to unify their activity in GA4.

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Solution

GA4 supports User ID tracking, allowing you to send a consistent identifier for authenticated users. You must send the user_id parameter with events and configure GA4 to use User-ID reporting identity.

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Step-by-Step Guide

Part 1: Send the User ID

You need to pass a unique user_id value when the user is logged in. This must be done in your GA4 configuration tag or global site tag.

Using gtag.js:

gtag('config', 'G-XXXXXXXXXX', {
  'user_id': 'USER_12345'
});

• Replace 'USER_12345' with a real identifier (e.g., a customer ID from your authentication system).
• This code should only run when the user is authenticated.

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Using Google Tag Manager:

1. Create a new User ID variable in GTM:
   • Go to Variables > New
   • Use a JavaScript Variable, First Party Cookie, or Data Layer Variable to capture the User ID
2. In your GA4 Configuration tag, under Fields to Set, add:
   • Field Name: user_id
   • Value: Your user ID variable
3. Save and publish your GTM container

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Part 2: Enable User-ID Reporting in GA4

1. Go to https://analytics.google.com
2. Select your GA4 property
3. Click Admin > Reporting Identity
4. Choose Blended or Observed:
   • Blended (recommended): Combines user_id, device ID, and modeling
   • Observed: Prioritizes user_id exclusively

Blended ensures fallback tracking when the user is anonymous.

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Testing Your Implementation

• Use Realtime and DebugView in GA4 to confirm the user_id is being sent
• Visit the site as a logged-in user and trigger a page view or event
• In User Explorer (Explore > User Explorer), look for activity tied to user_id

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Notes

• GA4 does not expose individual user IDs in reports, but you can analyze user behavior based on the ID internally (e.g., using BigQuery)
• Use hashed or anonymized IDs to protect user privacy
• The user_id must remain consistent and stable, and not PII (e.g., don’t use raw email addresses)