Engagement rate measures interactions (likes, comments, shares, saves, or GA4 engaged sessions) relative to reach or impressions.
The formula and even the included actions vary by platform and by context — a social media engagement rate (likes + comments + shares ÷ reach or followers) is a completely different calculation from GA4's engagement rate (engaged sessions ÷ total sessions). Treat "engagement rate" as a category of metrics, not one metric, and always check what's in the numerator before comparing two engagement rates.
High engagement doesn't guarantee business value — a meme-style ad can rack up comments and shares from people who were never in-market, while a plain, unglamorous product demo with lower engagement quietly drives the actual sales.
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