The two forms talk about future actions, and that’s why at first they seem equivalent. I am going to eat and will eat place the action after the moment of speaking. The difference is not in the basic tense, but in the way of presenting that action. Ir a + infinitivo sounds closer to a decision, a plan or a situation already headed. The future simple can sound more distant, more general or more formal, depending on the context. For that reason both forms can appear in similar sentences, but they do not convey exactly the same idea.

Use ir a + infinitivo when the action is already decided or when the immediate situation makes it very probable. The structure is ir + a + infinitivo. I’m going to call Marta says that the call is already thought. We’re going to go out now presents a near and concrete action. It also fits when you see clear signals of the moment: Va a llover if the sky is dark and the air changes. In all those cases, the sentence sounds tied to the present and to something that is about to happen.

Use of ir a infinitivo for nearby plans

Usage	Explanation
Already decided plan	Use ir a infinitivo when the action is already decided and sounds near.
Immediate intention	Use ir a infinitivo when you want to show an immediate intention.
Very near action	Use ir a infinitivo when the event seems very near in time.

The future simple tends to appear in broader predictions, without a previous decision as visible. Tomorrow it will rain on the coast presents a fact seen from outside the immediate situation. Next year I will travel more expresses a future idea less tied to the present moment. In many contexts, the future simple also sounds more careful or more formal than ir a + infinitivo. That’s why it appears often when talking about plans, changes or outcomes in a general way, without signaling much immediacy.

Use of the simple future for formal predictions

Usage	Explanation
Predicción general	Use the simple future for a more general or less concrete prediction.
Tono formal	Use the simple future when you want a more formal or written tone.
Distancia temporal	Use the simple future when the action sounds farther away or less immediate.

When you ask about someone else’s plan or intention, ir to + infinitivo is the natural form. What are you going to do after? asks for the immediate plan. Are you going to study tonight? asks about a concrete intention. You can also use it to find out a decision already made: Is he going to accept the job? In these questions, the focus is on what the other person thinks they will do, not on an abstract prediction. If you want to understand the speaker’s intention, ir a + infinitivo sounds direct and habitual.

Questions with ir a infinitivo about intention

Usage	Explanation
Question about a plan	Use ir a infinitivo to ask about a plan that someone intends to carry out.
Question about intention	Use ir a infinitivo to ask about the other person's intention.
Question about a decision	Use ir a infinitivo to confirm an already made decision.

The future simple doesn’t only serve to talk about future time. It can also express promise, warning, threat or emphatic intention. I will call you tomorrow can sound like a promise. You won’t arrive late again works as a warning or threat. I will do my best expresses commitment. In these uses, the speaker describes not only a future fact; they take a stance toward that fact. The value of the sentence depends on the relationship between the speakers and the tone of the situation.

Use of Futuro simple for promises and warnings

Usage	Explanation
Promise	Use Futuro simple to make a strong promise.
Warning	Use Futuro simple to give a clear warning.
Threat or emphatic intention	Use Futuro simple to express an emphatic intention or a threat.

The future simple can also express probability about the present. You’ll be at home doesn’t necessarily mean the person will be in the future; it means they are probably at home now. Could it be eight o’clock can be used to estimate the time with doubt. They’ll be about twenty years old expresses an estimate. This form is common when there isn’t total certainty and the speaker makes a reasonable guess. In this use, the future simple relates to inference, not to future time.

Use of Futuro simple for current assumptions

Usage	Explanation
Present probability	Use Futuro simple for an assumption about now when you are not sure.
Mental estimate	Use Futuro simple for an estimate based on clues.
Reasonable doubt	Use Futuro simple to show a reasonable doubt about the present.

In news, announcements and written texts, the future simple appears very often. The minister will travel today to Brussels sounds more journalistic than is going to travel. The meeting will begin at ten fits well in an announcement or program. In formal registers, this form helps present facts with distance and clarity. It is also useful in documents, press releases and headlines, where the style seeks precision and a certain objectivity. That’s why the future simple is easily associated with Register Formal and with informative texts.

Use of Futuro simple in written registers

Usage	Explanation
News	Use Futuro simple in news to present expected or announced events.
Formal text	Use Futuro simple in a more written or formal register.
Public announcement	Use Futuro simple when a statement sounds institutional or official.