I don't know whether there is a specific formula that makes a classic, but I think they should be books that everyone has heard of or are familiar with to some degree, regardless of whether or not they have read the book. I think classics should also always be suitable material for study in the classroom, whether that be secondary school, college, or university. I used to think a classic had to be literary, but don't think that is the case, because The Lord of the Rings is genre fiction, and that is a classic. Classics don't have to be old either. You can have modern day classics, and Harry Potter is probably one of them.