Bounce rate was simple: the percentage of sessions where someone visited one page and left. GA4 replaced it with "engagement rate." Neither metric tells the complete story.
What bounce rate actually measures
A "bounce" is a session with exactly one page view. The problem: a visitor who reads your 2,000-word article for 8 minutes and closes the tab registers as a bounce. They clearly engaged — the metric just doesn't capture it.
What engagement rate measures
GA4's engagement rate counts sessions as "engaged" if they lasted at least 10 seconds, had a conversion event, or had 2+ page views. Better proxy for actual engagement but still has blind spots.
What you should actually track
For content sites
Track pages per visit (is content discovery working?) and return visitor rate (are readers coming back?).
For SaaS and product sites
Track signup conversion rate via a page visit goal on your /welcome or /onboarding page. This is orders of magnitude more useful than bounce rate.
For e-commerce
Track purchase goal conversion rate as a page visit goal on your order-confirmation page.
The practical takeaway
A site with a 90% "bounce rate" but a 5% purchase conversion is doing fine. A site with a 10% "bounce rate" but a 0.1% purchase conversion has a real problem. Set up conversion goals for the actions that matter to your business — they tell the real story.