Update time: Dec 19, 2024
In website and app analytics, Client ID is a key identifier used by Google Analytics 4 (GA4) and other analytics platforms to distinguish different visitors.
What Is Client ID?
Client ID is a randomly generated identifier assigned to each unique visitor. It is used to identify user interactions that occur on the same browser or device.
In GA4, the Client ID can be generated via client-side tracking or server-side tracking and is stored in a first-party cookie.
Client ID does not contain any personally identifiable information (PII). However, it can be used to measure user behavior, sessions, and visit counts.
Note: Client ID is different from User ID. User ID is typically provided by a backend system and is used to associate the same user across multiple devices, while Client ID is only unique within a single device or browser.
What Is Client ID Used For?
- Distinguish unique visitors：Identifies each individual visitor to a website or app, preventing duplicate counting.
- Link user interactions：Connects multiple visits, events, pageviews, and conversions from the same device.
- Analyze user journeys：Supports session analysis, funnel analysis, and behavioral segmentation.
- Complement User ID：Acts as a fallback or supplement when cross-device User ID tracking is not available.
- Privacy-friendly identification：Uses an anonymous identifier, enabling user analysis without exposing personal data.
Client ID in Client-Side Tracking
In client-side tracking, the Client ID is typically generated as a random number combined with a timestamp, for example:
1197596843.1673515099
It is stored in the browser’s first-party _ga cookie.
A typical _ga cookie value looks like this:
GA1.1.1197596843.1673515099
_ga cookie structure
| Value | Description |
|---|---|
| GA1 | Version identifier |
| 1 | Domain level (cookie scope, default is top-level domain) |
| 1197596843 | Randomly generated unique number |
| 1673515099 | Timestamp indicating the first visit |
Key characteristics:
- Client ID is generated via JavaScript
- It can be read and used by client-side tools such as Google Tag Manager (GTM)
Client ID in Server-Side Tracking
In server-side tracking, the Client ID format is more complex and usually consists of two character strings plus a timestamp, for example:
F99Fd7gVuQuP93MZdJiEn07o/eZba6j9bAt8ETLsn8E=.1733280703
It is stored in the FPID cookie
which typically looks like this:
FPID2.2.F99Fd7gVuQuP93MZdJiEn07o/eZba6j9bAt8ETLsn8E=.1733280703
FPID cookie structure
| Value | Description |
|---|---|
| FPID2.2 | Version number |
| F99Fd7gVuQuP93MZdJiEn07o | Core user identifier |
| eZba6j9bAt8ETLsn8E= | Additional security or validation data |
| 1733280703 | Timestamp |
Key characteristics:
- Generated on the server side via HTTP responses
- Stored as an HttpOnly cookie
- Cannot be directly read by client-side tools such as GTM
Client-Side vs Server-Side Client ID
| Comparison Item | Client-Side (_ga) |
Server-Side (FPID) |
|---|---|---|
| Generation method | JavaScript-generated | HTTP response-generated |
| Cookie name | _ga |
FPID |
| Client-side readable | Yes (via GTM) | No (HttpOnly) |
| Format | Random number + timestamp | Two character strings + timestamp |
| Expiration | ~13 months | ~13 months |
Read more ：
- What is 「Effective user ID」 in GA4 User Explorer?
- Explanation of Cookie Values Used by Google Analytics 4
- How to Get Client ID to Variable in GTM？
- FPID: First Party Device ID
References