In GA4, the percentage of sessions that were not engaged; for the same nonempty session set, it is 100% minus engagement rate.
In GA4, bounce rate is the percentage of sessions that were not engaged. For the same nonempty session set, it equals 100% minus engagement rate. A session is not a bounce merely because it contains only one page view.
The classification depends on all the engagement criteria. A session can qualify through duration, a key event or multiple page or screen views. The source shorthand of a short visit with no key event is incomplete if the session still contains enough views to qualify.
Bounce rate is not a direct measurement of disappointment. Someone may complete a simple information task quickly, and measurement settings may affect classification. Avoid importing a definition from a different analytics product or an older reporting system without checking what the current metric actually means.
Takeaway: Explain the GA4 definition whenever bounce rate appears in a report. Investigate page purpose and tracking quality rather than treating every bounce as a failed customer experience.
Example
A report contains 150 sessions, of which 90 are engaged. The remaining 60 produce a 40% bounce rate. A single-page visit among the 90 may still be engaged if it met the duration or key-event condition.
What to check
For zero sessions, the percentage is undefined. Keep scope and engagement settings consistent in comparisons. Do not prescribe a universal acceptable bounce rate or assume reducing it necessarily increases sales.
For more detail, see GA4 engagement rate and bounce rate.