Reach
Reach is the number of unique users who may see posts containing the term you are tracking. Reach is calculated based on follower count.
For example, if you post a Tweet and share it with your 5,000 followers, reach for that post would equal 5,000, as it is the number of potential unique users who you are reaching with your post.
Impressions
Impressions are based on the total number of potential views of the term you are tracking. Unlike reach, impressions calculations also take into account how many times a specific user may have seen posts containing the term you are tracking.
For example, if you post 5 tweets with the term #sports, each of your 2,000 followers may have seen #sports 5 times, so your impressions would be 10,000. Therefore, Impressions is calculated based on: (Number of Unique Posts x Number of Unique Followers).
A single user can deliver multiple impressions. Impressions are captured based on your Twitter, YouTube, Instagram Business and Facebook data.
Platform capabilities for Impressions and Reach:
Twitter, LinkedIn Facebook and YouTube: Able to collect impressions Data
Instagram: Unable to collect impressions created by unauthenticated personal Instagram accounts and non-specified/unauthenticated Instagram Business accounts.. To specify instagram Business Accounts you want to track, go here. To authenticate an account go here. The Instagram API limits us with how much personal data we are able to collect which is why we require authentication.
Example of how Impressions and Reach are calculated:
If you have 500 followers and share 2 posts, your reach would be 500 but your impressions would be 1,000.
If you think the number is wrong, the first thing to look at is posts and users in your dashboard. If these numbers are equal, your reach and impressions will be equal.
If you have 500 followers and share 1 post, your reach would be 500 and your impressions would be 500.
When people start sharing multiple posts, your impressions will change. As a simple example, we will say each of the 500 people shared the post twice.
So, if 500 people each with 500 followers share a post once the reach would be calculated as 500 people x 500 followers = reach of 250,000.
Impressions, however would now be 500 people * 500 followers * 2 posts = 500,000 impressions.