This calculator grades an Instagram account against size-matched bands rather than one flat number, because engagement falls as an audience grows. For an account with 10,000 to 100,000 followers the bands are 1% at the bottom, 2% for average, 3.5% for good and 6% for excellent. Above a million followers the same four thresholds drop to 0.6%, 1.2%, 2% and 3.5%.
The gap between the two ends of that table is the whole reason a single benchmark misleads. An account under 1,000 followers needs 11% to be rated excellent; an account over a million needs 3.5%, which is 3.1 times lower. A 2 percent rate is below average for a 5,000-follower account and comfortably good for a 5,000,000-follower one, and it is the same 2 percent.
The rate itself is likes plus comments across the most recent posts, divided by followers. That is a recent-content measure, not a lifetime one, so it moves with what an account has actually posted lately - which is the honest way round, because a brand is buying the next post rather than the archive. Instagram publishes no engagement distribution of its own, so these bands are the model's stated thresholds and not a percentile of the platform.