Understanding Sessions in Google Analytics 4 (GA4): A Deep Dive

Introduction

In web and app analytics, sessions are the foundation of most user-level metrics. However, GA4 fundamentally redefines sessions compared to Universal Analytics (UA).

Where UA relied heavily on time-based rules, cookies, and pageview hits, GA4 is event-driven, focusing on user engagement and cross-device tracking. Understanding GA4 sessions is critical to correctly interpreting metrics like Active Users, Engagement Rate, Conversions, and Session Duration.

This guide explains what sessions are in GA4, how they are measured, and the key differences from UA, providing a strong conceptual foundation for analysis.

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What is a Session in GA4?

A session in GA4 is a group of user interactions (events) within a given time frame. Unlike UA, which relied on hits and pageviews, GA4 sessions are event-based, meaning every session is composed of one or more events.

GA4 Session Characteristics:

Feature	GA4 Behavior	Notes
Session Start	Triggered by session_start event	Every new session automatically generates this event
Session End	Implicit based on inactivity	GA4 uses a 30-minute default inactivity timeout, similar to UA
Cross-Device	Sessions can be stitched for user_id	One user may have multiple sessions merged across devices
Engagement	Only events matter, not pageviews	Sessions without user_engagement are still counted, but Active Users consider engagement

How GA4 Counts Sessions

In GA4, sessions are event-driven, with the following key rules:

1. Session Start (session_start)
   • Triggered automatically when a user starts interacting with a site or app.
   • Can also be triggered manually via event configuration.
2. Session Timeout
   • Default: 30 minutes of inactivity.
   • Can be adjusted in GA4 property settings.
   • If a user returns after timeout, a new session starts.
3. Midnight Reset / Campaign Change
   • Unlike UA, GA4 does not reset sessions at midnight.
   • Campaign source changes do not trigger a new session by default, but can be configured via parameters.
4. Cross-Device Sessions
   • GA4 uses user_id to stitch sessions across devices.
   • A single user interacting on mobile and desktop can have multiple sessions, but GA4 may report a single user session count depending on reporting identity settings.

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Session Metrics in GA4

Metric	Definition in GA4	Key Differences from UA
Sessions	Count of session_start events	UA counted sessions based on hits/pageviews; GA4 counts events
Engaged Sessions	Sessions lasting ≥10s, 1+ conversion event, or 2+ pageviews/screens	Introduced in GA4; helps measure meaningful engagement
Average Session Duration	Sum of engagement time / sessions	GA4 uses engaged time rather than total session time
Bounce Rate	Not in GA4; replaced by Engagement Rate	Bounce Rate = 1 − Engagement Rate

GA4 emphasizes quality over quantity, focusing on engaged sessions, which better reflect user behavior.

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Session Example: Event Flow

Imagine a user interacts with your site:

1. 10:00 AM – User lands → session_start triggered
2. 10:01 AM – Views home page → page_view event
3. 10:05 AM – Clicks product → select_item event
4. 10:10 AM – Leaves browser idle for 35 minutes → session ends (timeout)
5. 10:45 AM – Returns and interacts → new session_start event triggered

Notes:

• GA4 automatically counts two sessions.
• If user_id is implemented, these sessions may be stitched for user reporting depending on identity settings.
• Only the second session contributes to new engagement metrics if the first session had no engagement events.

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Key Differences Between GA4 and UA Sessions

Feature	Universal Analytics (UA)	GA4
Event Model	Hit-based	Event-based
Session Start	Pageview or screenview	session_start event
Session Timeout	30 min default, resets on campaign change	30 min default, configurable, ignores campaign changes by default
Engagement	Bounce Rate	Engaged Sessions (≥10s, 1+ conversion, 2+ pageviews/screens)
Cross-Device	Cookie-based	user_id stitching possible
Campaign Attribution	Resets session	Sessions continue regardless of campaign change unless configured

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Best Practices for Working with GA4 Sessions

1. Understand engagement events – Sessions alone may overcount low-quality interactions.
2. Use session_start for session-level queries – Avoid using page_view events to approximate sessions.
3. Implement user_id – Enables cross-device session stitching and accurate user-level reporting.
4. Adjust session timeout if necessary – For long user interactions, extend beyond 30 minutes.
5. Focus on engaged sessions for analysis – These better reflect meaningful user activity.

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TL;DR

• GA4 sessions are event-based, not hit-based like UA.
• A session is initiated by a session_start event and ends after inactivity or timeout.
• GA4 introduces engaged sessions and engagement-focused metrics, replacing bounce rate.
• Cross-device sessions are stitched if user_id is implemented.
• For accurate analysis, use session_start events, apply engagement filters, and account for timezones and session timeout settings.