What is Session Start in Google Analytics?

If you’ve spent any time inside Google Analytics 4, you've undoubtedly come across the session_start event in your reports. At first glance, it seems simple enough - it’s when a session starts. But this single event is the cornerstone of how GA4 measures user engagement and marks a significant shift from how Universal Analytics used to work. Understanding it properly is your first step toward getting truly meaningful insights from your data.

This article will break down exactly what the session_start event is, how GA4 uses it to define a session, how it differs from the old way of tracking, and how you can use it to better analyze your website or app traffic.

What Exactly Is the session_start Event?

The session_start event is an automatically collected event in Google Analytics 4 that fires whenever a user begins a new session on your website or app. Think of it as the digital equivalent of a welcome mat. The moment a user lands on your site for the first time or returns after a period of inactivity, GA4 logs a session_start event to mark the beginning of their visit.

This is central to GA4's event-based measurement model. Unlike its predecessor (Universal Analytics), where sessions were a "container" for hits like pageviews and transactions, GA4 treats everything as an event. A page view is an event, a button click is an event, a purchase is an event, and starting a session is also an event. This approach provides a much more flexible and user-centric view of behavior.

A Quick Trip Down Memory Lane: Universal Analytics vs. GA4 Sessions

One of the most common points of confusion for anyone migrating to GA4 is why their session count doesn't perfectly match the numbers from their old Universal Analytics (UA) property. The reason is simple: they are measured differently.

In Universal Analytics, a session was a collection of hits and would end for one of three reasons:

• Time-based expiration: By default, after 30 minutes of user inactivity.
• End of day: Sessions would automatically end at midnight in your reporting timezone, and a new one would start.
• New campaign source: If a user arrived via one campaign, left, and then immediately came back via another campaign (e.g., from Google Ads then from an email link), UA would end the first session and start a new one.

In Google Analytics 4, the rules are much simpler. A session starts with the session_start event and ends only after a period of inactivity. By default, this is 30 minutes, but there are no exceptions for the time of day or a change in campaign source. If a user is active on your site at 11:59 PM and continues browsing past midnight, it's still counted as one continuous session in GA4.

This streamlined approach means your session count in GA4 may be slightly lower than in UA, but it often provides a more accurate picture of a user's single, uninterrupted browsing experience.

The Rules of Engagement: When Does a New Session Actually Start?

Since GA4 relies solely on a timeout to determine a session's end, understanding this mechanism is vital. When a user lands on your site, GA4 starts a timer. Each time the user interacts with your site (e.g., loads a page, clicks a button, scrolls), the timer resets.

If the user stops interacting and the timer reaches its limit (again, 30 minutes by default), the session is considered over. If that same user returns to your site after the session has timed out, Google fires a new session_start event, and a new session begins.

Let's use a practical example:

• A user clicks a link from your newsletter and lands on your blog at 2:00 PM. A session_start event is recorded.
• They read the article for 10 minutes, click to another page, and watch an embedded video. Their session is active.
• At 2:15 PM, they get a phone call and walk away from their computer, leaving your website open in a browser tab.
• If they return at 2:30 PM (15 minutes of inactivity) and click a link, their original session continues because the 30-minute timeout limit wasn't reached.
• However, if they return at 2:50 PM (35 minutes of inactivity), their previous session has expired. When they click a link now, GA4 will log a new session_start event.

How to Adjust Your Session Timeout Settings

While 30 minutes is the standard, it might not be ideal for every website. If you host long-form content, videos, or complex applications where a user might be inactive for longer periods, you may want to extend this timeout. Conversely, if your site is for quick transactions, a shorter timeout might be appropriate.

You can easily change this setting by following these steps:

1. In your GA4 property, click on Admin (the gear icon) in the bottom-left corner.
2. Under the Property column, click on Data Streams and select your web data stream.
3. Scroll down and click on Configure tag settings.
4. Under Settings, click Show all to expand the options.
5. Click on Adjust session timeout.
6. Here, you can set the hours and minutes for both the session timeout and the timer for engaged sessions. Choose your duration and click Save.

Adjusting this helps ensure your data accurately reflects how users genuinely interact with your specific content.

Putting It to Work: Finding and Using session_start Data

The session_start event isn't usually a metric you analyze directly. Instead, it's the foundation for several of the most important metrics in your GA4 reports that tell you how much and how engaged your traffic is.

Here are the key metrics derived from it and how to use them:

1. Sessions

This is the most straightforward derivative. The "Sessions" metric you see in reports like Traffic acquisition is simply the total count of session_start events for the selected period. This is your high-level metric for understanding traffic volume.

• How to use it: Trend this metric over time to understand your site's overall growth. Segment it by traffic channel (e.g., Organic Search, Paid Social, Email) to see which marketing channels are most effective at driving visits.

2. Engaged sessions

This is one of GA4's most valuable improvements. An "engaged session" is a session that either:

• Lasted longer than 10 seconds (you can adjust this), OR
• Included a conversion event, OR
• Had at least 2 pageviews or screenviews.

This metric filters out the low-quality traffic from users who land on your site and leave almost immediately without doing anything. It helps you focus on visits that are actually meaningful.

• How to use it: Compare total sessions to engaged sessions. If a paid campaign is driving 1,000 sessions but only 50 are engaged, it's a sign that the ad targeting or landing page may be disconnected from user intent.

3. Engagement rate

Engagement rate is simply the percentage of sessions that were engaged sessions (Engaged sessions / Total sessions). It's the successor to "Bounce Rate" and provides a much more positive and nuanced view of user interaction.

• How to use it: Look at the engagement rate for your top landing pages. A page with a high number of sessions but a low engagement rate might not be meeting visitor expectations, giving you a clear signal to optimize it.

Going Deeper: Parameters Passed with session_start

Like every event in GA4, session_start comes packaged with useful bits of information called parameters. Two of the most important ones it helps generate are:

• ga_session_id: This is a unique identifier assigned to each session. It gets attached to every subsequent event within that same session (like pageviews, clicks, and conversions). This is incredibly powerful because it allows you to group all the actions a user took during a single visit.
• ga_session_number: This parameter counts how many sessions a particular user has initiated. If a user is visiting your site for the very first time, their session_start event will have a ga_session_number of 1. If they return tomorrow, their next session_start event will have a count of 2. This is an excellent way to analyze the behavior of new vs. returning users from the very start of their journey.

While you might not use these parameters in standard reports, they are the backbone of GA4's Explore reports, where you can build deep-dive analyses like funnels and path explorations to see exactly what users are doing visit by visit.

Final Thoughts

The session_start event in GA4 is far more than just a counter for visits. It's the foundational block that enables a more flexible, user-focused analysis of website and app engagement. By understanding how it works, what defines a session, and which metrics are built from it, you can move past high-level traffic numbers and start uncovering true insights into your user behavior.

While GA4 offers powerful data, getting these kinds of clear answers still requires clicking through multiple reports and knowing exactly what you're looking for. Instead of spending your time manually building reports to compare sessions versus engagement rate across different campaigns, we let you connect Google Analytics and simply ask plain-English questions like, "What were my top 10 landing pages by engagement rate last month?". We built Graphed to create live dashboards that answer your questions for you in seconds, turning hours of analysis into a quick conversation and getting you back to the work that matters.