Sometimes there may be no difference at all between both these questions. At the same time, there may be situations where there is a slight difference between them.
“How are
you?” is a bland greeting for someone you haven’t seen for a while, while “How
are you doing?” may be an actual inquiry. The latter is more common when there
is some expectation that the subject might not be doing well.
For
example: “Hey, haven’t seen you in ages! How are you?” versus . . . “How are
you doing: is the new baby still keeping you up all night?”
—cricketswool
on English Language
& Usage Stack Exchange
—
How
are you doing?
—
I
am doing well. Or
—
I’m
good.
“I’m doing
good” is grammatically incorrect. If you say “I’m doing good” it could mean
you’re doing some charity work or something positive.
You’re doing
well or you are good.