TL;DR:
Session duration is the length of a single visit, measured from the first pageview to the last recorded interaction in that session. Because analytics often cannot time the final page (there is no next event to measure against), average session duration is an estimate, and GA4 replaced it with 'average engagement time'. Read it as a trend, not an exact stopwatch.
Session duration answers a simple-sounding question (how long did a visit last?) that is surprisingly hard to measure exactly. It is one of the most quoted engagement metrics and one of the most misunderstood, because of what happens on the last page of a visit.
How session duration is measured
A session is a single visit, a run of activity from one visitor before they leave or go idle. Duration is the time between the first event and the last event in that session. The catch is the final page: classic analytics measures time by the gap between events, and the last page has no next event, so its time often counts as zero. That is why a one-page visit historically showed a duration of zero even if the person read for five minutes.
What GA4 changed
GA4 moved away from 'average session duration' toward 'average engagement time', which measures the time a page was actually in focus rather than the gap between events. It is a more honest attempt at the same idea, but it means GA4 numbers are not directly comparable to older Universal Analytics session-duration figures.
Treat average session duration as a relative signal (up or down versus last week, or between two pages) not as a precise stopwatch. The last-page problem makes the absolute number an estimate in most tools.
Open Analytics shows engagement alongside the journey and source behind it, cookielessly.
See how it worksFrequently asked questions
- What is a good average session duration?
- There is no universal number; it depends entirely on the site's purpose. A long-read blog or a documentation site will show longer sessions than a landing page that sends people onward quickly. Compare it against your own trend and against similar pages, not against a generic benchmark.
- Why is my session duration zero or very low?
- Usually the last-page problem: many tools time a page by the gap to the next event, and the final page of a visit has no next event, so its time counts as zero. Single-page visits are the extreme case. GA4's 'engagement time' tries to fix this by measuring focus time instead.
- Is session duration the same as time on page?
- No. Time on page is for a single page; session duration covers the whole visit across all pages. Both share the last-event measurement problem, which is why absolute values are estimates and trends are the more reliable read.