Understanding Client ID in GA4 (Client-Side Tracking and Server-Side Tracking)

Table of Contents

Update time: Dec 19, 2024

You may see different structures of effective user ID in the User explore in GA4 exploration, as shown below:

Display logic of effective user ID:

If User ID is deployed, display User ID
Otherwise deployed Device ID
- For APP, the Device ID is the instance ID.
- For Websites, the device ID is the Client ID.
  - For Client-Side Tracking, the Client ID is a random number plus a timestamp, eg: 1197596843.1673515099
  - For Server-Side Tracking， the Client ID is two strings of characters plus a timestamp, eg: F99Fd7gVuQuP93MZdJiEn07o/eZba6j9bAt8ETLsn8E=.1733280703

This is because the Client ID is different in Client-Side Tracking and Server-Side Tracking.

Client-Side Tracking

For Client-Side Tracking, the Client ID is stored in the cookie _ga. The structure of _ga is as follows:

Note: 1197596843.1673515099 is the Client ID.

The detailed explanation is as follows:

Value	Description
GA1	Version number, which represents the version of the cookie format that’s being used.
1	Domain Level, In fact, GA4 generates all 1.
1197596843	Random Number, 10 digits
1673515099	Timestamp, the timestamp when cookie _ga was created

For Server-Side Tracking, the Client ID is stored in the Cookie FPID. The structure of FPID is as follows:

The value of FPID is as follows:

FPID2.2.F99Fd7gVuQuP93MZdJiEn07o/eZba6j9bAt8ETLsn8E=.1733280703

Note: F99Fd7gVuQuP93MZdJiEn07o/eZba6j9bAt8ETLsn8E=.1733280703 is also called Client ID

If you don't understand, You can leave a comment below.