'going to ' is more certain and expresses nearer future than in the case of using 'will'.

You won't get a clear answer to this question because, depending on the context, sometimes they mean the same thing. There are no clear rules only guidelines. The following is my understanding. Plans and intentions. From the point of view of the speaker "will" carries more certainty than " going to" because it expresses the idea of something that is planned, decided, or promised. Examples: 1) Your boss says: a) You will receive a raise next month. = The decision has been made, it is almost a promise. b) Your are going to receive a raise next month. = if all goes according to plan between now and then.. For this reason formal documents always use "will" instead of "going to". 2) An announcement over the loudspeaker. a) The museum is going to close in five minutes.= According to their present plan, but you can probably linger for a few more minutes. b) The museum will close in five minutes. = This is an unchangeable schedule, so if you don't leave now you may be locked in. 3) Bill: I lost my wallet a) Mary: I'll get the check! = immediate future intention, almost a promise Bill: Thank you! That's very kind of you. b) Mary: I 'm going to get the check. = It's my present intention. Bill: Are you sure? I can call a friend. Predicting the future Someone says to you: a) It will rain in an hour . = The speaker is convinced, but on what evidence? The listener doesn't know. You might well ask, "How do you know?" b) It is going to rain in an hour. = If present conditions continue then it will rain. In this case, for the speaker a) is more certain, while for the listener b) carries more weight because it is based on present evidence.

The both express the same degree of certainty. IN fact, their difference has NOTHING to do with certainty or nearness or distance. It has to do with decision making! I'm going to join the club = decision already prior to speaking If the club is good I'll join up. decision not already made (waiting to see if the club is good) That is the difference between the two.

You're heading for mistakes if you define the two based on certainty. "Will" is a predicted future event; you're pretty sure of it happening. "Going to" suggests a progress towards the future action (and as Cherry has pointed out, could express the near future), or at least a clear intention to do the action.

Compare two people who are expressing the same idea: "I will go swimming with you later" "I am going swimming with you later" One is more of a statement of fact, while the other is a little open-ended, saying that you WANT to (will) or plan to. Am, a state of being, is more solid, but in many cases the two terms are entirely identical, or can't be told apart.