Session ID dimension: scoping events to one visit
Session ID is the dimension that ties a burst of events together into one session. In GA4 it is derived from the ga_session_id parameter set when a session_start event fires, and it pairs with the client or user ID to be unique. A session ends after a timeout of inactivity (30 minutes by default), so the same person returning later gets a new session ID — which is why session counts respond to the timeout setting.
What this means
GA4 starts a session when the session_start event fires and records ga_session_id (a timestamp-based value). Every event in that visit carries the same session ID, so the dimension lets you reconstruct the order and content of a single visit.
To be globally unique, the session ID is combined with the client or user ID — the same numeric session ID can recur across different users.
Timeouts and what splits a session
Sessions end after a configurable period of inactivity — 30 minutes by default in GA4. Cross a midnight boundary or campaign change and behaviour varies by tool. The practical effect: a visitor who steps away and returns 40 minutes later starts a new session with a new ID, inflating session counts relative to people.
Unlike a logout, nothing the user does deliberately ends a session; the clock does.
- Derived from ga_session_id at session_start
- Unique only when paired with client/user ID
- Bounded by an inactivity timeout (30 min default)
How it appears in analytics and logs
A session ID groups events from one continuous visit. More sessions per user can simply mean the timeout elapsed between visits; it is not inherently a sign of churn or re-engagement.
Diagnostic use case
Use session ID to reconstruct what happened within one visit, while knowing the inactivity timeout, not a logout, is what bounds a session.
What WebmasterID can help detect
WebmasterID groups first-party events into sessions without cross-site identity, so per-visit analysis works without third-party cookies.
Common mistakes
- Treating the session ID as globally unique without the user/client ID.
- Reading more sessions per user as engagement rather than timeouts.
- Assuming a session ends on logout rather than on inactivity.
Privacy and accuracy notes
Session ID is a pseudonymous, time-scoped token, not a personal identifier on its own. WebmasterID scopes session identifiers first-party and never joins them across sites.
Related pages
- Client ID dimension: the device-scoped pseudonym
Client ID is the device-scoped pseudonymous identifier analytics generates to recognise a returning browser. In GA4 it is typically stored in a first-party cookie and underpins the user and session counts. It is not a person: clearing cookies, switching browsers, or using a new device all create a fresh client ID, which is why user counts drift upward and cross-device journeys split without a User-ID.
- New vs returning dimension
The new vs returning dimension classifies a visitor as new (no prior recorded visit) or returning. The classification depends on a persistent client identifier surviving between visits. When cookies or storage are cleared, browsers cap identifier lifetime, or a user switches devices, returning visitors are recounted as new — so this dimension systematically tilts toward 'new' and should be read with that bias in mind.
- The session_start event
The session_start event marks the beginning of a visit. In GA4 it is fired automatically the first time a user is active in a new session, and it underpins every session-scoped metric. Understanding when a session begins — and the timeout that ends it — explains why session counts behave the way they do across midnight, campaigns, and idle periods.
- Event Explorer
Trace the events that share one session ID.
Sources and verification notes
- Google Analytics Help — [GA4] Automatically collected events (session_start)Documents session_start and ga_session_id behaviour.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.