One of the many uses of the future tenses in English is to make predictions – to suggest, or guess, something is likely to happen in the future. We usually make predictions with the future simple forms of will (It will be a good day.) or going to (Our team are going to win this game.). There is a subtle difference between the two forms.

Using the will future form for predictions

The will future form can be used for predictions not based on evidence. These predictions are often based on opinion, so will sounds more personal, or determined. This can include guesses, judgements based on character or assertions of faith.

These predictions often use adverbs such as probably or definitely to show the strength of opinion.

I will win this game. (This shows determination, but it is not a known fact.)
The shop will probably lose money this month. (A guess or estimate.)

Using the going to future form for predictions

If you replace will with the going to form in the above examples, the prediction sounds more definite.

I am going to win this game. (It is almost certain.)
The shop is probably going to lose money this month. (It is likely.)

Predictions demonstrated with going to are therefore similar to planned events. They are future events that we have reason to believe are very likely, or inevitable. It may be based on present or past evidence.