If you want the standard American English answer, then you'd say "well" - NOT "good". However, the vernacular (ie, everyday speech amongst friends) "good" is acceptable in informal situations. However, just know that some people will correct you if you use the vernacular, thinking that you don't know the proper usage. So, unless you know the people and are comfortable with them, I'd suggest using "well". Simple answer: VERY casual situations = use good. Uncertain situations or formal ones = use well. Hope this clears it up. Good luck!

The correct answer is "I'm fine" or "I'm well", however Americans have taken to saying "I'm good". This though is considered incorrect in British English (although with the influence of American films and TV shows, it doesn't stop some people saying it).

People say both, but I tend to say and hear "I'm good" more often than "very well." Hope this helps :)

And if you are asked "Are you allright / OK?" the reply will be "I'm fine, thanks"

I'm very well thanks! And you? Very formal and proper. But you could could say either.