Will and be going to

A Introduction

Emma: It's my birthday soon. I'll be twenty next Friday.

Aunt Joan: Oh, really? Are you going to have a party?

Emma: I'm going to have a meal in a restaurant with a few friends.

Aunt Joan: That'll be nice

WILL

Will has a neutral meaning. We use it to talk about facts in the future.

I'll be twenty next Friday.

The spacecraft will come down in the Pacific Ocean tomorrow morning.

BE GOING TO

We use be going to for an intention, something we have already decided to do.

We're going to have a meal.

Tom is going to sell his car.

Will does not express an intention.

It's her birthday. She's going to have a meal with her friends, NOT She'll have a meal.

But we often use be going to for an intention and will for the details and comments.

We're all going to have a meal. There'll be about ten of us. ~ Oh, that'll be nice.

As well as be going to, we can use the present continuous.

We're going to drive/We're driving down to the South of France. ~ That'll be a long journey.

Yes it'll take two days. We'll arrive on Sunday.

B Decisions and intentions

WILL

We use will for an instant decision or agreement to do something.

There's a postbox over there. I'll post these letters.

You still haven't put those shelves up, Trevor. ~ OK, I'll do it tomorrow.

Trevor is deciding now.

BE GOING TO

Be going to means that we have already decided.

I'm going out. I'm going to post these letters.

You still haven't put those shelves up, Trevor. ~ I know. I'm going to do it tomorrow.

Trevor has already decided.

C Predictions

WILL

We can use will for a prediction about the future.

I think United will win the game.

One day people will travel to Mars.

BE GOING TO

We use be going to for a prediction when we see from the present situation what is going to happen in the future.

There isn't a cloud in the sky. It's going to be a lovely day.

This bag isn't very strong. It's going to break.

It is often possible to use either form in a prediction. For example, we can also say I think United are going to win the game. Usually be going to is a little more informal and conversational than will.