The default to know when someone has unfollowed you is by checking if they still appear on your list of followers. If there are no longer on that list, it means they unfollowed you. However, this manual method can be stressful and time-consuming. Thankfully, there are third-party apps that can tell you who has unfollowed you on Instagram, and you do not have to go through the stress of the method above.
Some apps to try are InsTrack, Followers for Instagram, FollowMeter, and InstaFollow. Most of these apps are available for both Android and iPhone devices. To use these apps, you need to login to your Instagram account and grant permission to the app; it will automatically track the unfollowers and new followers.
The normal way to follow who unfollowed you on Instagram is to check your followers’ list, but if you are following too many people, it will be hard to spot the users who decided to unfollow your account. However, there are several applications that you can download for free which can help you easily track your new followers and unfollowers.
You can choose unfollowers.com, justunfollow.com, insta.friendorfollow.com, unfollowgram or followers+. I am using followers+ because it is easy to use. All you have to do is connect your Instagram account on the application, and you will be able to see: New followers, have unfollowed me, are not following back, users who blocked me, and deleted likes and comments (but this option is only available if you upgrade to a premium plan).