Cross-Device User Tracking with GA4’s user_id Parameter

GA4’s user_id parameter enables accurate cross-device user tracking by assigning a unique, persistent identifier to logged-in users, allowing their interactions across multiple devices and sessions to be unified into a single user journey. This means when a user logs in on different devices, GA4 recognizes them as the same person rather than separate users, improving user count accuracy and providing a holistic view of behaviour.

Key points about using the user_id for cross-device tracking in GA4:

• User ID is a custom identifier that you generate and assign to users when they authenticate (e.g., log in). This ID is sent with every event to GA4, ensuring persistence across sessions and devices.

• GA4 automatically persists the user ID once set, but best practice is to send the user_id parameter with every event to guarantee consistent tracking.

• User ID tracking requires users to be logged in or otherwise identified, so it only applies to authenticated users, but it offers the highest accuracy among GA4’s identity methods.

• GA4 combines user_id with other identity signals like Google Signals (for Google account users) and device IDs to provide blended or observed reporting identities, which enhance cross-device insights.

• To implement, you typically add the user_id parameter in your GA4 configuration tag via Google Tag Manager or directly in your site/app code.

• Enabling Google Signals alongside user_id further improves cross-device tracking by leveraging Google account data, but user_id remains the core method for logged-in user tracking.

• Cross-device reports in GA4 (such as Technology reports and User Explorer) allow you to analyse unified user journeys across devices using the user_id data.

In summary, the user_id parameter in GA4 is essential for precise cross-device tracking of authenticated users, enabling businesses to see a unified user journey across multiple devices and sessions, thereby improving analytics accuracy and marketing insights.