Effective User ID Explained in GA4

Updated: December 19, 2025

In GA4 User Explorer, you may notice different structures for the Effective User ID depending on your tracking setup, as shown below:

Display Logic of Effective User ID

The Effective User ID is determined by the following logic:

• If a User ID is implemented: GA4 displays the User ID.
• If no User ID is set: GA4 falls back to the Device ID, which differs based on platform:
  • App: Device ID = Instance ID
  • Website: Device ID = Client ID
    • Client-side Tracking: The Client ID is generated as a random number combined with a timestamp. Example: 1197596843.1673515099
    • Server-side Tracking: The Client ID consists of two character strings plus a timestamp. Example: F99Fd7gVuQuP93MZdJiEn07o/eZba6j9bAt8ETLsn8E=.1733280703

BigQuery Mapping

In BigQuery exports, the Effective User ID corresponds to the user_pseudo_id field. This allows you to analyze users consistently across both GA4 reports and raw event data.

Read more ：

• Understanding Client ID in Google Analytics 4
• Various IDs in Google Analytics 4