“Will” and “Going to”

Both “will” and “going to” can be used to indicate a future action or a prediction.  However, they differ slightly.  We often use “will” for the first time that we think of doing the action; we use “going to” when the action has already be determined, as we can see in the following example:

Riding his bicycle, a little boy named Tom runs over a sharp rock.  It punctures his tire.  He walks his bike home, sees his mother, and asks her, “Mom, could you please fix my tire?”

His mother replies, “I’m a bit busy, but I will fix it in ten minutes, Tom.”

The boy leaves, sees his father, and says, “I have a flat tire; I’d like mom to fix it.”  Before Tom can relate that he has already asked his mom, the father calls to her, “Can you fix Tom’s tire?”

She replies, “I am going to fix it in ten minutes.”

She uses “will” when she replies to Tom because she did not know about the flat tire and made a plan for the first time.  She uses “going to” when she replies to her husband because she has already made a plan to fix the tire.

Having written that, I should note that this rule does not always hold and that many speakers and writers use “will” and “going to” interchangeably. 

Resources

The BYU English Language Center has a simple PDF that outlines some of the differences.

Portland Community College has a quiz.