Dress code for a concert

Eh for classical pieces, I'd say at the minimum go for some dressier pants/slacks (no jeans, shorts, or cargos) and a dressier shirt as well of course, like a button up or maybe a polo? Depends on what your style is, but generally you want to look a bit nicer than casual clothes, without going for the full on suit and tie deal. I mean you could go that far, but it's hardly required, and usually you'd be told if it was.

For other concerts, it depends entirely on where you're going and who you're going with. If you're going to a rock concert, you'd probably be just fine with jeans and a t shirt or whatever casual style you go for. If you were going to a metal concert with a mosh pit, you might want to wear clothing you don't mind getting torn/ripped/etc. Though again, it really does depend a lot on who you're going with and what you're going to see.