Update time: December 18, 2025
In Google Analytics 4 (GA4), a session represents a group of user interactions that take place on your website or app within a defined time window. In simple terms, a session reflects one visit by a user and can include page views, events, e-commerce actions, and other interactions.
While the idea of a session may sound familiar from Universal Analytics (UA), GA4 handles sessions very differently under the hood. Understanding how GA4 defines, tracks, and counts sessions is essential for interpreting reports correctly and avoiding common misunderstandings.
What Exactly Is a Session in GA4?
In GA4, a session is not a fixed container of hits, but rather a logical grouping of events that share the same session ID (ga_session_id).
A session typically includes:
- Page views or screen views
- Custom events
- Engagement events
- E-commerce interactions
All of these events are tied together as long as they occur within the same session window.
When Does a Session End?
By default, a GA4 session ends after 30 minutes of inactivity.
This timeout period can be adjusted in GA4 Admin settings, up to a maximum of 7 hours and 55 minutes.
Example：
- User visits your site at 1:00 PM
- Interacts for 3 minutes
- Leaves the tab open without further activity
The session ends at 1:33 PM
When Does a Session Start?
A session in GA4 starts in one of the following situations:
- First visit：A user visits your website or opens your app for the first time.
- New visit after inactivity：A user returns after the previous session has timed out (by default, after 30 minutes of inactivity).
When a session starts, GA4 automatically triggers a session_start event.
How GA4 Tracks Sessions
When a session starts, GA4 generates and attaches two key session parameters to events:
ga_session_id
Represents the session ID，Technically, it is a timestamp indicating when the session began
To uniquely identify a session outside GA4, it is recommended to combine:user_id or user_pseudo_id with ga_session_id
Important caveat: Because ga_session_id is a timestamp, it is not globally unique. The same timestamp can occur for different users, which is one reason session counts in GA4 are estimated rather than exact.
ga_session_number
Indicates how many sessions a user has had up to the current one
Example:
- 3 = the user’s third session
- 5 = the user’s fifth session
Where Session Information Is Stored
GA4 stores session-related data in a cookie named _ga_<container-id>, each part is described as follows:
The detailed explanation is as follows:
| Value | Description |
|---|---|
| GS | Fixed identifier (Google Session). |
| 2 | Analysis version (commonly 1 or 2). |
| 1 | Domain level (almost always 1). |
| 1673933110 | Session ID/Current Session Start Time（aka：ga_session_id ） |
| 12 | Sessions Count（aka：ga_session_number） |
| 0 | Engagement session flag (1 = engaged, 0 = not engaged). |
| 1673933110 | Current Timestamp |
| 60 | Countdown，60-second countdown, cookie and user data matching/syncing when Google Signals is enabled |
| 0 | User-ID status (1 = logged in with User-ID set). |
| 0 | Enhanced Client Id, abbreviated as ecid, is a string of numbers that only has value when tracked on the server side tracking or enable 「User-ID and user-provided data collection」 |
| riYSmWx3uWvuJEDkCvwJCnvET7QcvFq7kg | This field is available when Google signal data collection is enabled and indicates the Google signal or the associated ID of the Google signal. |
Important: For the first request of a session, the Session ID and Current Timestamp are identical, because the Session ID represents the session start time.
Read More：GA4 Cookie Values Explained
How GA4 Calculates the Number of Sessions
According to Google’s official definition:
GA4 calculates the number of sessions that occur on your site or app by estimating the number of unique session IDs.
Sessions in GA4 are estimated metrics, similar to Active users and Total users，read more: Estimated Metrics in GA4: Sessions、Active Users and Total Users
Because Session IDs are timestamps and can repeat (not guaranteed to be unique)。
You may notice discrepancies such as:
Total sessions in reports ≠ sessions shown in the report header
Key Session-Related Metrics in GA4
GA4 introduces a stronger focus on engagement, not just raw session volume.
- Engaged sessions：An engaged session is a session that lasts longer than 10 seconds, has a conversion event, or has at least 2 pageviews or screenviews.
- Engaged sessions per user：The average number of engaged sessions per user.
- Engagement rate：Engagement rate = engaged sessions / total sessions, this metric replaces the traditional “bounce rate” concept from Universal Analytics and provides a more meaningful view of user interaction quality.
Final Thoughts
Sessions still matter in GA4—but they are no longer the core unit of measurement they once were in Universal Analytics. GA4 is fundamentally event-driven, and sessions are now a derived, estimated construct built on top of events.
