Depends why you want to learn the theory. If you play an instrument enough, you'll pick up lots of theory, probably enough for an average amateur, naturally, ju

Depends why you want to learn the theory. If you play an instrument enough, you'll pick up lots of theory, probably enough for an average amateur, naturally, just by learning lots of pieces. Piano is better for this than most I believe as its easy to see what's going on and play chords (vs say, wind instrument that plays just 1 note at a time). If you wanna be a really good amateur (presumably not a pro, as people normally has to start as a kid to do that), or, want some knowledge to help with jazz improv, then I'd say aim for ABRSM Grade 5 theory, possibly do grade 3 or 4 first. If you google My Music theory, there's a ton of free stuff on that website. you can do ABRSM exams online. Good way of checking your knowledge and getting a certificate of proof :). I know some people say you don't need to study theory for jazz improv. Personally, cos I don't play piano or guitar, I found studying theory helps fill in gaps of knowledge eg on chords