Average Session Duration is the mean time users spend in a session on your site or app. In plain terms: take the length of every session within a period, add them up, divide by the number of sessions. It’s a classic web analytics KPI to gauge visit depth alongside pages per session, engagement rate, and bounce rate.
Formula (core idea):
Average Session Duration = Σ (session durations) ÷ (number of sessions)
How a session duration is measured:
A session’s duration is typically the timestamp of the last recorded hit minus the first recorded hit (think first to last pageview or event). A single-hit session (a bounce) often computes as 0 seconds because there’s no “next” hit to mark elapsed time. That’s why this metric correlates with bounce rate and can look artificially low on content with many single-page visits.
Mini example (3 sessions):
| Session | Hits (simplified) | First → Last hit | Duration |
|---|---|---|---|
| #1 | Page A → Page B → Event | 10:00 → 10:45 | 00:45 |
| #2 | Page C → Page D → Page E → Event | 14:05 → 18:20 | 04:15 |
| #3 | Page F (bounce) | 21:10 → 21:10 | 00:00 |
Average Session Duration = (00:45 + 04:15 + 00:00) ÷ 3 = 01:40
Why analysts use it
- Quick pulse on visit depth across channels, campaigns, or content types.
- Useful for comparing cohorts when paired with conversion rate and exit rate.
- Signals whether long-form content is actually being consumed (with caveats).
Limitations to keep in mind
- Last-hit problem: No subsequent hit, no time accrued. Consider adding meaningful engagement events (scrolls, plays, form progress) to reduce zero-time sessions.
- Not “time on page”: That’s a different metric—see Time on Page. Session duration aggregates many hits; a long last page still won’t add time without another hit.
- Easily gamed: Over-instrumenting trivial events inflates time without improving value. Track only actions that represent real engagement.
Practical tips
- Compare Average Session Duration by source/medium and content category, not in isolation.
- Watch it together with engagement rate and pages per session to understand whether “longer” also means “better.”