If you’re not sure whether to use ‘will’ or ‘going to’, just do this.
Ask yourself ‘Did I plan the action at the time of speaking or did I plan the action before I spoke?’
If you planned the action before you spoke, use ‘going to’.
If you planned the action at the time that you spoke, use ‘will’.
For example, ‘I’m going to Japan next year.’ I used ‘going to’ here because I made that plan before I said it. I’ve already bought plane tickets. I’ve already booked a hotel.
Oh my phone’s ringing! I’ll pick it up! Hello. Yeah sorry, I can’t talk now. I’m filming this video. I’ll call you later. Okay bye.
I said ‘I’ll pick it up’ and I said ‘I’ll call you later’. I said this because I made those decisions when I spoke. I didn’t plan to do them before I spoke. I made the plan when I was speaking.
So if you’re not sure whether to use ‘will’ or ‘going to’, use this rule. Normally, you will be right.