We will follow the International Standards Organization (ISO) system for register designations. In that system, middle C (the first ledger line above the bass staff or the first ledger line below the treble staff) is C4. An octave higher than middle C is C5, and an octave lower than middle C is C3.
What note number is middle C?
Middle C (the fourth C key from left on a standard 88-key piano keyboard) is designated C4 in scientific pitch notation, and c′ in Helmholtz pitch notation; it is note number 60 in MIDI notation.
What note is an octave above middle C?
An octave above Middle C is C5, an octave below Middle C is C3, etc. In short, the C's octave range is in play until the next C changes the octave register.
