You migrated from Universal Analytics to GA4 and your bounce rate jumped from 30% to 70%. Sessions dropped by 40%. Pageviews doubled. None of it makes sense until you understand that GA4 changed the fundamental definitions of these metrics — not the data collection, the definitions themselves. This guide explains every major metric change so you can build apples-to-apples comparisons.

Bounce Rate: Completely Redefined

UA Bounce Rate: percentage of sessions with only 1 pageview. A user who reads your entire blog post for 10 minutes and leaves = 100% bounce.

GA4 Bounce Rate: percentage of sessions that were NOT engaged. An engaged session requires 2+ pageviews OR 10+ seconds OR a conversion event. The same 10-minute reader = engaged session = 0% contribution to bounce rate. GA4 bounce rate is typically lower for content sites, potentially higher for single-page landing pages with no conversion.

Sessions: When They Start and End

UA Sessions: ended after 30 minutes inactivity OR at midnight OR on source change (new campaign mid-visit). GA4 Sessions: end only after 30 minutes inactivity. No midnight reset. No source-change reset. Result: GA4 typically reports fewer sessions than UA for the same traffic. Campaign-driven session inflation is eliminated.

Users: New vs Returning

UA used a 2-year cookie. GA4 uses a 2-year client_id cookie too, but Safari ITP limits this to 7 days. Expect GA4 to show higher new user percentages than UA because returning Safari users are miscounted as new after 7 days. Implement server-side cookies (see our guide) to fix this.

Conversion Rate: Which Version Are You Comparing?

UA conversion rate = goal completions / sessions. GA4 shows two rates:

• Session conversion rate: conversions / sessions — closest to UA’s metric
• User conversion rate: converting users / total users — always lower, can appear 2–3x lower than UA

Always compare UA session conversion rate to GA4 session conversion rate — not to the user conversion rate, which is a different calculation.

Pageviews vs Views

GA4 doesn’t have “pageviews” by default — it has “Views” combining page_view events. For standard sites, Views ≈ UA Pageviews. For SPAs with virtual page view tracking, Views may be higher than UA pageviews if the UA implementation missed virtual pages.

Metric Mapping Reference

• UA Bounce Rate → GA4: 100% minus Engagement Rate (not a direct equivalent)
• UA Sessions → GA4 Sessions: GA4 usually lower (no midnight/source splitting)
• UA Goal Completions → GA4 Conversions (if same events are marked)
• UA Avg. Session Duration → GA4 Avg. Session Duration (methodology similar)
• UA New Users → GA4 New Users: GA4 typically shows higher percentage due to ITP

Related: GA4 Session Timeout, GA4 Cross-Device Tracking.