going to / will. When do we use it?

The <going to> form used for prediction

The <going to> form can express the speaker's feeling of certainty.
The time is usually not mentioned, but the action is expected to happen in the near or immediate future:

Look at those clouds! It's going to rain!

Listen to the wind. We're going to have a rough crossing.

It can be used in this way after such verbs as <be sure/afraid>, <believe>, <think>:

How pale that girl is! I am sure/I believe/I think she is going to faint.

Comparison of <be going to> (used for prediction) with <will> (used for probable future)

<will> is a common way of expressing what the speaker thinks, believes, hopes, assumes, fears etc. will happen.

It will probably be cold/I expect it will be cold.

Tomatoes will be expensive this year/I'm sure tomatoes will be expensive.
<will> and <going to> are therefore rather similar and often either form can be used.

It will take a long time to photocopy all the documents = It is going to take a long time to photocopy all the documents.

But there are two differences:

1. <be going to> implies that there are signs that something will happen, <will> implies that the speaker thinks/believes that it will happen.

2. <be going to> is normally used about the immediate/fairly immediate future, <will> doesn't imply any particular time and could refer to the remote future.

For example, <The lift is going to break down> implies that it is making strange noises or behaving in a strange way; we had better get out on the next floor.
<The lift will break down> implies that this will happen some time in the future.

Source: A Practical English Grammar, A.J. Thomson, A.V. Martinet