MIDI Note Number Converter

In short MIDI note numbers are whole numbers from 0 to 127, one per semitone, and the two that matter are 60 for middle C and 69 for A4 at 440 hertz. Everything else follows from those: the frequency of note n is 440 × 2^((n − 69) ÷ 12), so note 0 is C−1 at about 8.18 Hz — below the range of human hearing — and note 127 is G9 at about 12,544 Hz, near the top of it. The 88 keys of a piano occupy notes 21 to 108. Two things trip people up. The first is octave numbering: this page uses the convention where middle C is C4, which is what MIDI itself and most notation software use, but some manufacturers label the same note C3 and a few call it C5. The note number never changes, only the name printed next to it, so when two pieces of software disagree about the octave the MIDI number is the thing to trust. The second is that MIDI notes are integers with no room between them. A pitch bend or a microtonal tuning cannot be expressed as a note number at all — it needs a separate pitch-bend message or a tuning table, which is why quarter-tone music has always sat awkwardly with MIDI.