There are two main formulas, and they answer different questions. Engagement by followers (interactions ÷ follower count) is the classic vetting metric but punishes large accounts whose posts reach beyond their following. Engagement by reach (interactions ÷ accounts reached) better reflects how compelling the content actually was to the people who saw it.
Benchmarks shift by platform and size: smaller accounts almost always post higher rates, and TikTok rates run far above Instagram's. Rough vetting heuristics in influencer marketing treat 1 to 3% (by followers, on Instagram) as ordinary and above that as strong, but any fixed number ages quickly. The metric's real value is comparative: against your own baseline, and against similar-size accounts in the same niche. Inflated follower counts with bottom-tier engagement is the classic bought-audience signature.
