What's the difference between "will" and "to be going to" in English?

Question

When do I say "I will" and "I am going to"?

Answer 1

The two CAN be used with the same meaning, but generally

"WILL" is used for:
a) promises
- "(If elected), I will raise your taxes!"

b) offers/volunteering to do something
- "I'll (I will) help you clean up this mess."

c) demanding/forcing someone to do something
- "You WILL finish your homework before you go to the movie!"

"(BE) GOING TO" is used for:
intentions, decisions made before speaking (for something already planned):
- "I'm going to come home now."


EITHER FORM can be used for predictions,

"Who do you think will be (is going to be) the next President?"

But "going to" appears especially to be suited to predictions 'based on physical evidence', e.g., "It's boing to rain" or "(I can see) this is going to take us all day!"


See the following pages for more explanations, and examples of all these uses.
http://www.englishpage.com/verbpage/simplefuture.html
http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/futwil.htm
http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/gofut.htm
http://esl.about.com/library/grammar/blfuture.htm

Answer 2

The problem is harder than this. There is also the possibility of using the present tense for the future, and also the progressive future, andso we have:

I will go
I will be going
I am going to go
I am going
and even I am going to be going

These are not equivalent. F.ex. you can say My sister is getting married tomorrow or will be getting, but My sister will get married tomorrow would mean something quite different.
Something like 'I will make sure she gets married or else!'.
Also, very occasionally we use the simple present for the future, as in Where does the duke dine tonite? But this is rare nowadays. However, this is a vast topic. The rules are not simple. Best source might an up to date book on English grammar.

Answer 3

bruhaha and Alaric both gave good answers, particularly their points about formality, emphasis, and intention.

Another major difference between them is that "will' is really a modal, and thus is tenseless (though we use it to express the future), while 'be going to" is a semi-modal phrase that can be used in the present or past tense as well, and also in counterfactual (conditionals and so on).

I was going to do it
I will be going to do it
I had been going to do it
I would have been going to do it
I am going to do it
I'm always going to do it, I just never do do it

Answer 4

Saying ''will'' indicates more of in the far future, but "going to" indicates something closer to now.

compare:

"I will go to the bookstore"

"I am going to the bookstore"

Don't worry there's not really a nuance to it.

Answer 5

The time frame.
I will be rich some day (not right away)
I am going to be rich tonight...i am getting my pay check.
(very soon)