There’s the midi system, where middle C is C5 I think. or there’s the standard Helmholz one, which is easy. There the octave below middle c is cdefgab and the octave from middle C up is c’d'e’f'g’a'b’.

All you really have to remember is that it breaks at C. so re-entrant C is g’c'e’a’ and low 4th (sequential) C is gc’e'a’. The “Canadian” tuning, low 4th D, is ad’f#’b', and high4th d is a’d'f#’b’.

For comparison, guitar is EAdgbe’ and mandolin/fiddle is gd’a'e”. See how it works?