How many people see FB posts?

[Pac M]

Im not talking about those posts that you have to pay to promote or anything. Im talking about those times when you just post something for fun, like how many people see that? Theres this one guy I occasionally see stuff from in the news feed and I never like his posts or even "know" him in real life (we went to high school together a long time ago, weren't even real friends)

So I wonder how many random people are seeing the stuff I post? Any specific formula for figuring this out?
Thanks for the replies guys, I think I should have worded this differently though. I know about privacy settings and stuff like that, what I want to know is a rough estimate of how many people see it if I make the post "public" and do not pay FB anything to promote it?