Will vs. Be going to

Will vs. Be going to: Similarity

We use both will and be going to when we talk about any future event. Example:

I am going on a tour tomorrow.

I will call you when I reach the destination.

Will vs. Be going to: Difference

We use will when we talk about something that is either always true or has been decided or announced just now. 

We use be going to when we talk about something that is in motion or in process because it has been decided long before.