Engagement rate
Engagement rate is interactions divided by audience size, expressed as a percentage. The public version divides average likes plus comments by followers; the analytics version divides by reach, measuring how content performs with people who actually saw it. It shrinks mechanically as audiences grow, so it should only ever be benchmarked within a follower tier and niche.
Worked example
A creator with 20,000 followers averaging 700 likes and 40 comments per post has (700 + 40) / 20,000 × 100 = 3.7% by followers.
Related: Reach · Fake followers · Save rate
Full context in the guides, or measure it yourself with the free tools.