Will and going to both talk about the future, but they signal different things to the listener. The core difference is timing: will points to a decision or thought happening now, while going to points to a decision or observation that already exists.
Spontaneous decisions vs pre-made plans
This is the most practical split between the two forms.
Use will when you decide something at the moment of speaking. The decision did not exist five seconds ago.
Use going to when the decision was made before the conversation. You already have a plan.
A useful test: if you can add I just decided before the sentence, use will. If you can add I decided earlier, use going to.
Predictions: opinion vs evidence
Both forms can make predictions, but they come from different places.
Will expresses a prediction based on opinion, belief, or general knowledge. There is no visible proof -- you simply think this is what will happen.
Going to expresses a prediction based on something you can already see, hear, or measure. The evidence is right in front of you.
The phrase I think is a strong signal for will, because it marks an opinion. Present evidence (look, watch out, listen) is a strong signal for going to.
When both forms work
In some situations the difference fades, and either form is natural.
General predictions about the distant future. When you are guessing about something far away, with no strong evidence either way, both are fine.
Promises. Both forms can carry a promise, though will is slightly more common here.
Warnings. When the evidence is clear and the event feels immediate, going to is more natural. But will is not wrong.
Offers, promises, and refusals
These three situations strongly prefer will because they involve a reaction to something happening right now.
Offers -- you see a need and respond.
Promises -- you commit to something in the moment.
Refusals -- someone or something will not cooperate.
Present Continuous as a third option
English has a third way to talk about the future: the Present Continuous (am/is/are + -ing). It works when the plan is a confirmed arrangement -- something fixed with another person, often with a specific time or place.
The difference from going to is subtle. Going to signals intention -- you plan to do it. Present Continuous signals arrangement -- it is already organized, and other people are expecting you.
In practice, the two often overlap: I'm going to fly to Paris on Friday and I'm flying to Paris on Friday are both natural if the ticket is booked. The Present Continuous simply sounds more definite.
Quick decision guide
When choosing between the three forms, one question usually settles it.
Am I deciding right now, in this conversation? Use will: I'll have the chicken.
Did I decide before this conversation? Use going to: I'm going to visit my parents this weekend.
Is it a confirmed arrangement with a time or place? Use Present Continuous: I'm meeting Anna at noon.
Am I predicting from evidence I can see? Use going to: The sky is black -- it's going to storm.
Am I predicting from opinion or belief? Use will: I think the economy will recover.
Does either form feel natural? Then either is fine. This happens more often than textbooks suggest.
Common mistakes
Using will for pre-made plans
Using going to for spontaneous reactions
Mixing up prediction types
Using will for confirmed arrangements
Using going to for offers