A lot of piano students wonder how much music theory they need to know if they want to become advanced pianists. You can actually get away with knowing very little music theory. Where the real benefit of having and advanced background in music theory is in memorizing pieces and developing really great interpretations. If you can intuitively understand why a composer makes a certain key change in a piece, then you will be able to memorize that piece much more easily than someone who doesn’t understand that concept.
At a minimum, you need to be able to read music very well to become a great pianist. You should also have a basic understanding of the different key signatures and how they relate to one another. If you can learn all the scales and chords within each key signature, you will put yourself at a great advantage over other people. You basically want to become fluent in reading music, to the point where it becomes second nature and you can learn notes very quickly. If you can’t do this, you will spend too much time trying to figure out what notes to play and you won’t ever advance very far.
If you don’t know music theory very well, it helps if you can at least recognize patterns. If you can tell that a certain piece repeats a section from the beginning in a different key, that piece of knowledge will be very helpful to you when trying to memorize your pieces. There are always tons of patterns in classical music, you just need to be on the lookout for them. If you are trying to memorize a passage, try to think of ways you can think about the passage on a deeper level than just what the notes are. See if you can tell how they relate to each other. It might be difficult to do this at first, but after a while you should be able to do it rather easily. This is the best way to get around learning music theory at an advanced level.
So how much music theory do you need to know to be a great pianist. Well, the answer is “it depends.” There are certainly ways around it, so if you are good at reading music and can find patterns in music, you can be a really great pianist without knowing music theory all that well.