Avg. Session Duration
IntermediateAverage session duration is the mean gap between the first and the last recorded event in a session, computed across every session in the selected range.
The metric is built from event timestamps, so the time spent on the final page cannot be measured at all: no later event arrives to close the interval. The direct consequence is that any single-event session is recorded as zero and drags the overall average down. Tools genuinely disagree here: Universal Analytics used this last-minus-first definition, while GA4 dropped it and reports average engagement time per session instead, counting only the time the tab was in the foreground and active, so the two numbers never match on the same site. The common misreading is to treat a longer average as deeper engagement, when confusion, slow pages and an abandoned background tab all push the same number up.
How it is calculated
Average session duration = total duration of all sessions ÷ number of sessions, where the duration of one session is the timestamp of its last event minus the timestamp of its first event
The numerator sums the seconds of every session in the range and the denominator counts those same sessions, including single-event sessions that enter as zero; excluding them inflates the result sharply. The unit is seconds or minutes. If the tool sends heartbeat pings, single-page sessions are no longer zero and the same underlying data yields a larger figure under a different definition.