Engagement rate is engaged sessions / total sessions, where a session counts as engaged when any one of three things is true: it lasts longer than 10 seconds, it fires a conversion event, or it includes at least two pageviews or screenviews. The 10-second threshold is the GA4 default, not a fixed law. GA4 promoted engagement rate to the primary site-quality metric in its default reporting surface around the time it replaced Universal Analytics, taking the slot bounce rate used to occupy.
The 10-second floor is configurable in the property’s data-stream settings, and operators commonly raise it to 30 or 60 seconds to harden the signal against drive-by bots and accidental clicks. Engagement rate and bounce rate are roughly inverse, but engagement rate + bounce rate ≈ 100% only holds when the 10-second timer is the sole separator between the two. Once conversion events fire reliably and multi-page sessions are common, the relationship loosens.
Because the metric collapses three signals into one, a drop is not self-diagnosing. An 8% week-over-week fall could mean shorter sessions, fewer conversions, fewer multi-page sessions, or some mix. Unpack the components before deciding what to act on. Treat engagement rate as a paid-traffic-quality signal more than a content-quality one — segment by source/medium (the channel side of attribution) before reading a low rate as a page problem.