The formula
engagement = interactions ÷ followers × 100
What it means
On followers is the classic metric and the one used for collaboration rates. On reach is more honest, because it measures the people who actually saw the post: on large accounts reach is usually a small fraction of the followers, so the second percentage comes out considerably higher.
How to work it out by hand
- Add up every interaction: likes, comments, saves and shares
- Divide by the number of followers
- Multiply by 100
What is worth knowing
The rate falls as the account grows. Below 10,000 followers, 3 % or 4 % is normal; above 100,000, 1.5 % is already good. Saves and shares carry more weight than likes as far as reach goes, because the algorithm reads them as a signal of real value.