User Engagement

You open a page, read a couple of lines, and a message comes in. You switch to another tab, reply, scroll a bit, and come back 10 minutes later. How long were you on the page?

For GA4, only the time your page was in front counts. The 10 minutes on the other tab add nothing, even though the page never closed.

That's why engagement time isn't session duration. Your session can last 20 minutes while your engagement time stops at 5.

GA4 doesn't send this time second by second. It keeps a running count and attaches it to the next event, in a parameter called engagement_time_msec.

When you switch away, leave the page or close the tab, it sends what's left in a user_engagement event, with the time in the same parameter.

In your reports, the total appears as User engagement, and the averages as Average engagement time per active user and Average engagement time per session.

So a long session with little engagement time usually means a tab left open, not a careful reader.