Short answer
Use going to for a decision you already made and for a prediction you can see coming: I'm going to call her tonight (decided this morning), Look at those clouds, it's going to rain. Use will for a decision made as you speak, a promise or an offer: The phone is ringing, I'll get it.
| will | be going to | What changes |
|---|---|---|
| I'll have the salad. | I'm going to have the salad. | will: you decide while reading the menu. going to: you knew before you sat down. |
| It will rain later. | It's going to rain later. | will: your opinion about the sky. going to: the clouds are already there. |
| I'll help you move. | I'm going to help you move. | will: an offer made this second. going to: it was arranged days ago. |
| She'll pass the exam. | She's going to pass the exam. | will: you believe in her. going to: you have seen her practice scores. |
| I'll call you back. | I'm going to call you back. | will: a promise. going to: an item already on your to-do list. |
Now put it to work: 40 questions on the key distinction, will (spontaneous decisions, promises, opinion predictions) vs be going to (plans made earlier, evidence-based predictions).
Use 'will' when the decision happens as the speaker talks. Use 'be going to' when the decision was already made earlier. Five answers need will and five need going to.
| Situation | will ✓ | be going to ✓ |
|---|---|---|
| Decision made NOW | ✅ I'll take it. | ❌ |
| Decision made BEFORE | ❌ | ✅ I'm going to take it. (already decided) |
| Opinion prediction | ✅ I think it will work. | ❌ |
| Evidence prediction | ❌ | ✅ Look! It's going to rain. (clouds visible) |
| Promise | ✅ I'll be there. / I won't forget. | ❌ |
Going to points backwards to something that already exists: a decision you took earlier, or evidence in front of you. Will points to this second: the decision, the offer or the promise is being made as you speak. That is why English says "Look at those clouds, it's going to rain" but "I think it will rain tomorrow".
For a plain future plan, yes. "I'll see her on Friday" and "I'm going to see her on Friday" both work, and nobody will correct you. The choice starts to matter when there is visible evidence, which needs going to, or a promise, an offer or an on-the-spot decision, which needs will.
Ask yourself where the prediction comes from. If you can point at something happening right now, use going to: "She's gone pale, she's going to faint." If it comes from your opinion, your experience or a guess about a distant future, use will: "I'm sure prices will rise next year."
Gonna is how going to is pronounced in fast speech, and it is normal in songs, films and messages between friends. Write going to in emails, essays and exams. Also note that gonna only replaces going to before a verb, never before a noun: you can say I'm going to Paris, not I'm gonna Paris.
The free grammar test takes about fifteen minutes, gives you a CEFR level and lists the topics you keep getting wrong, each with a link to the lesson that fixes it. No account, no payment.
Take the free grammar test