Bounce rate vs. engagement rate

Same coin, two faces. Old Universal Analytics called a session a bounce if it fired exactly one hit, so someone who read a 2,000 word article for eight minutes and left counted as a bounce. GA4 flipped the definition: a session is engaged if it lasts 10 seconds or more, or racks up two or more pageviews, or fires a conversion event. Inside GA4, and only there, bounce rate is then simply 100 percent minus engagement rate. The two are complements because GA4 defines one as the leftover of the other; that is not true of Universal Analytics bounce rate or of whatever your other tool means by the word.

Reach for engagement rate when the page's job is to hold attention (blog, docs, landing page), and treat bounce as a diagnostic rather than a grade. A high bounce on a contact page where the phone number sits above the fold means the page worked. A high bounce on a pricing page usually means it did not.

Gotcha: both numbers are trivially gameable. Add a scroll-depth event or a 3 second timer and your bounce rate collapses overnight while nothing about the site improved. If you compare across time or across tools, check that the engagement definition did not quietly change underneath you.