Hi” and “Hey” are the both sound formal?

i think is not formal...its casual...
correct me if im wrong

One is formal and one is casual, the word "Hi" is often used as formal words for example, we use hi for some older people or teachers.

While "Hey" is more casual for example, we saw our friends and we say "hey", they will reply back. But using a "hey" towards some people (for example, towards our teacher) could be quite un-polite.