“Will” vs. “Going To”

“Will” and “going to” are both used in some pretty similar ways. Both are used to talk about the future. You can say “I will cook dinner tonight,” and “I’m going to cook dinner tonight.” The meaning of these two statements is almost exactly the same. But there are some subtle differences. And in other cases, will and going to can be used in very different ways.
 

Will vs. Going to: Times when it’s better to use “will”

Will is a much better word choice than going to in a number of situations.

Long term predictions

If you are making a prediction about something far in the future, will is the more common wording in English.

• Examples: The world will end 100 years from now. There will be another presidential election in four years.Notes: In this example, if you use going to instead, the statement will sound wrong — or at least awkward — to most native English speakers. “The world is going to end 100 years from now” just doesn’t sound right. Similarly, it’s better to say “There will  be another presidential election in four years,” compared to “There is going to be  another presidential election in four years.” Going to simply doesn’t have a sense of “distant future” in the same way that will does.

Requests

Requests absolutely have to use will. Going to is completely incorrect in this case.

• Example: Will you help me with my homework? Will you join us for dinner?
  
  Notes: “Going to help me with my homework?” and “Going to join us for dinner?” are so ungrammatical that a native speaker would have trouble understanding these. You could say “Are you going to help me with my homework?” or “Are you going to join us for dinner?” But these sentences don’t sound like requests. They sound like yes/no questions about future action. And if you try to frame such questions as requests, you’ll seem very rude and demanding.

Offers of help

If you are offering to help, will makes for a better, easier-to-understand statement than going to.

• Examples: Someone’s knocking on the door; I will let them in. I will help you cook dinner.Notes: If you say “I’m going to let them in,” your statement sounds stiff and a little unnatural. Your tone can also seem rude. “I’m going to let them in” can make it sound like letting someone in is a difficult task you resent doing. It can also sound as if you won’t allow anyone else to let the person in. In the same way, “I am going to  help you cook dinner” sounds like you’re demanding the right to help someone cook, or complaining that you have to help cook.

Complicated verb tenses

In complicated verb tenses for talking about the future, going to makes the grammar simply too complex — too many words and syllables.

• Examples: I will have been studying for 60 days by the end of the month. They will be sleeping in a hotel tonight. She will have finished her college degree by next spring. For the examples above, try substituting will for am going to or are going to. You’ll find that going to makes the sentences much harder to say. Harder to understand too!

Times when going to is better than will

There are also some situations where it’s better to say going to, and will doesn’t work so well. There are really only two times when going to is the more common construction.

Short term predictions

If you are making a prediction about something very near in the future — in a matter of minutes, hours, days, or sometimes weeks — going to is more common than will.

• Examples: It’s going to rain soon. The boss isn’t going to like this.Notes: In the first example, you can also say “It will rain soon,” but this changes the meaning of the sentence. “It will rain soon” sounds more like an absolute certainty than a prediction. In the second example, if you say “The boss will not like this,” the tone is again slightly more forceful. This second sentence is an interesting example though. Because upsetting the boss is a serious, forceful situation, going to and will are almost exactly the same here. But with normal, non-forceful predicitons, going to is always a better fit.

Plans

When you discuss plans, going to is the more appropriate construction. Will doesn’t work quite as well for talking about future plans in English.

• Examples: I’m going to get a haircut on Friday. She told me she is going to apply to Harvard.
  
  Notes: Here, will changes the meaning and sounds “off” in both examples. In the first example, “I will get a haircut on Friday” has a rude tone. It’s the kind of thing you would say angrily to someone who is bothering you about getting a haircut. And in the second case, if you say “She told me she will apply to Harvard,” there is a hint of disbelief. By saying will instead of going to, you suggest that her talk of applying to Harvard is a prediction that might not come true, rather than a plan that she’ll actively carry out.

A note on going to as a future tense form vs. “going to” as a phrasal verb

One of the confusing things about going to is that it isn’t always used as a will-like future tense form. Sometimes “going to” is actually a phrasal verb. This can be confusing because you can actually create some very similar sentences using future form going to and phrasal verb “going to.”

• Example 1:
  
  Future form:  He is going to fight in a war (predicting that he will fight in a war soon.)
  Phrasal verb:  He is going to war (he is physically relocating to a war zone so he can fight).
• Example 2:
  
  Future form:  I am going to watch a movie this weekend (future plan to watch a movie).
  Phrasal verb:  I am going to the movie theater this weekend (physically travelling to the movie theater in the near future, presumably to watch a movie).