When to use "what about you?" and when to use "how about you?" ?
When somebody says:
-- Hi there, HOW are you?
I say:
-- Hi, i'm fine, thanks, HOW about you?
Is it the same if I say: "what about you?" in that situation?

Question

When to use "what about you?" and when to use "how about you?" ?
When somebody says:
-- Hi there, HOW are you?
I say:
-- Hi, i'm fine, thanks, HOW about you?
Is it the same if I say: "what about you?" in that situation?

Answer

In this situation the phrases seem to be equal.

I would guess that whether one uses 'how' or 'what' would be based on colloquial speech.

Gramatically, I prefer "How are you?"

However, on a scale of most formal to least formal I would rank it this way:
"I'm fine, how are you?" most formal
"I'm fine, how about you? less formal
"I'm fine, what about you?" least formal