What is a session in PostHog?

A run of activity from one person with no gap longer than thirty minutes. PostHog derives sessions from the event stream rather than recording them, which is why session numbers behave differently from users and pageviews.

How do I count sessions in PostHog with HogQL?

SELECT count() FROM the sessions table, bucketing on min_timestamp so a session that spans midnight is counted on the day it started.

Why did my PostHog session count drop suddenly?

Often a background poll. If your app sends events on a timer, sessions never reach the thirty-minute gap that ends them, so many visits merge into one long session. Adding heartbeat tracking is the usual cause.

Are sessions the same as visits or users?

No. One person can produce many sessions in a day and many pageviews in a session. Sessions count runs of activity split on a thirty-minute gap, users are de-duplicated people, and pageviews count every load.

How to measure sessions in PostHog

Q: What is the most useful thing to do with a session count?

Divide pageviews by it. Depth rising means people are reading more. Depth rising while sessions fall is usually a navigation problem, with the same people hunting for something that used to be easy to find.

What is a PostHog session?

A run of activity from one person with no gap longer than thirty minutes. PostHog does not record sessions; it derives them from the event stream, and everything odd about session numbers follows from that.

The query

PostHog exposes a sessions table in HogQL, which saves you stitching events yourself.

SELECT toUnixTimestamp(toStartOfDay(min_timestamp)) AS bucket, count() AS value
FROM sessions
WHERE min_timestamp >= toDateTime({from}) AND min_timestamp < toDateTime({to})
GROUP BY bucket
ORDER BY bucket

min_timestamp is when the session started, and bucketing on it means a session that spans midnight is counted on the day it began. That is the convention every analytics tool uses and it is worth knowing you are accepting it.

The thirty-minute rule, and what it does to your numbers

Somebody reading a long article, going to make tea, and coming back after thirty-five minutes is two sessions. Somebody who leaves a tab open in the background and clicks once an hour is many sessions.

Neither is a bug. It is the definition doing its job, and it means session counts are sensitive to how your product is used in a way that pageviews and users are not. A product people keep open in a tab all day produces session counts that have very little to do with how many times they decided to use it.

Sessions move when nothing moved

Three things change session counts without any change in traffic.

A background poll. If your app sends an event on a timer, sessions never end and your session count collapses. People discover this after adding heartbeat tracking.

A tracking change. Firing more events in the gaps bridges what would have been two sessions into one.

Identity merging. When PostHog merges two persons, their sessions merge too, which can change historical figures.

So a session series is the least stable of the standard metrics, and reading a step in it as a behaviour change is usually wrong.

Pageviews per session is the useful form

Sessions on their own say little. Divided into pageviews they say how deep a visit goes.

-- Two queries, divided client-side. Keep the window identical.
SELECT count() FROM events WHERE event = '$pageview' AND ...
SELECT count() FROM sessions WHERE ...

Depth rising is people reading more. Depth rising while sessions fall is usually a navigation problem, with the same people hunting for something that used to be easy to find.

Sessions are not visits, and not users

Three different numbers that get used interchangeably in conversation and should never be in a chart.

	What it counts
Sessions	Runs of activity, split on a thirty-minute gap
Users	De-duplicated people over the window
Pageviews	Every page load, repeats included

One person can produce many sessions in a day and many pageviews in a session. Label the axis.

Or read it off the notch

Notch Metrics reports PostHog sessions as sessions started in the window, bucketed hourly, daily, weekly or monthly in your PostHog project's own timezone, with a sparkline and a comparison against the previous window.

Place it beside pageviews and active users and the three describe the visit rather than just counting it. Everything PostHog reports, or download Notch Metrics.