Will & Be Going To

When we want to express an activity in the future time, we usually use "will" or "be going to". Both of them have the same meaning. Now Let's see the specific function of "will" and "be going to".
1. When you want to predict something which will happen in the future time( prediction), you

can use both "will" and "be going to".

eg : Look at the cloudy sky! it is going to rain.
According to the weather report, it will rain today.

2. When you want to do something by your own willingness, you can use "will" to express it.
eg : Y : "somebody is knocking the door"
W : "I will open it."

3. To express a prior(main or more important) plan, you can only use"be going to".
eg : A : "Why did Mr. Smith packed all his clothes?"
B : "He is going to leave to New York.