Update time: September 5, 2025
What is Sessions？
Key Points About Sessions in GA4
When a Session Starts
- A user visits your website or opens your app for the first time.
- A user returns to your site or app after a previous session has expired (typically after 30 minutes of inactivity).
When a Session Ends
By default, a session ends or times out after 30 minutes of user inactivity, though this duration can be adjusted in GA4’s Admin settings (up to a maximum of 7 hours and 55 minutes). For example, if a user visits your site at 1:00 p.m., interacts for 3 minutes, and then leaves the tab open without further activity, the session will end at 1:33 p.m.
session_start
In principle, there is only one session_start for one session.
But it will actually appear that a session has multiple session_starts： Some CMP tools will clear the cookie, causing _ga_<container-id> to generate a new value, and session_id will have a new value, so it is regarded as a new session.
There is also a session_start of 0, which may be caused by midnight. If the user visits the website on the night of January 1 and leaves directly on January 2, then for January 2, session_start is 0; there may be other reason.
In addition, there is a special case: the session has only one event of session_start, and no other pageview events.
Session Tracking
When a session starts, GA4 automatically collects a session_start event and generates a session ID (ga_session_id) and session number (ga_session_number) via the session_start event.
- ga_session_id：Session ID is a timestamp of when a session began. To analyze different sessions outside of Google Analytics, consider joining the user_id or user_pseudo_id with the session_id to get a unique identifier for each session.（The number of sessions is the timestamp, so it is not unique, so the number of sessions in GA4 is smaller than the actual number）
- ga_session_number：Session number identifies the number of sessions that a user has started up to the current session (e.g., a user’s third or fifth session on your site).
These information are stored in a cookie named _ga_<container-id>, each part is described as follows:
The detailed explanation is as follows:
| Value | Description |
|---|---|
| GS | Fixed String: Fixed Identifier |
| 1 | Analysis Version, The value is 1 or 2, and there are two versions, corresponding to the two screenshots above. |
| 1 | Domain Level |
| 1673933110 | Session ID/Current Session Start Time |
| 12 | Sessions Count |
| 0 | Engagement Session, 1 means engagement session, 0 means no |
| 1673933110 | Current Timestamp |
| 60 | Countdown，60-second countdown, cookie and user data matching/syncing when Google Signals is enabled |
| 0 | Login and set User ID, 1 means login and set user id, 0 means no |
| 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. |
For the first request of the session, the Session ID and Current Timestamp are the same, because the Session ID is the Current Session Start Time.
Read More：Enhanced Client ID (ecid) in Google Analytics 4
Calculation of the Number of Sessions
The official definition of the number of sessions is:
GA4 calculates the number of sessions that occur on your site or app by estimating the number of unique session IDs.
Note that it is an estimate, and the session is an estimate, read more: Estimated Metrics in GA4: Sessions、Active Users and Total Users, so there will be GAP, such as:
The total number of sessions in different dimensions is not equal to the number of sessions in the head, and there will be gaps.
Note：The number of sessions is based on the deduplicated session_id, and the session_id is actually a timestamp, which will be repeated, so the number of sessions in GA4 is low.
Key Metrics
The session-related indicators in GA4 are as follows:
- 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
