Guide

A440 and MIDI 69

Unless you change the reference, A4 is 440 Hz. That is the default on the note frequency calculator. MIDI note 69 is A4 at whatever reference you set.

Equal temperament on this site uses one formula: f = A4 × 2^{((midi − 69) / 12)}. Each semitone multiplies frequency by the twelfth root of two. A4 at 440 Hz is 440.00. A5, twelve semitones higher, is 880.00.

Some rooms sit at 442 Hz. Some people prefer 432 Hz. The frequency page accepts a reference between 400 and 480 Hz so those values are explicit. Changing A4 moves every other note with the same formula. It does not retune a piano in the next room, and it is not a substitute for a hardware tuner on stage.

Note names are C, D, E, F, G, A, and B. In some countries the note B is written H; this site uses B. Chromatic names prefer sharps when you move upward.

The reverse direction — hertz to a note — finds the nearest MIDI number, then reports cents sharp or flat of that pitch. 440 Hz at A4 = 440 is A4 and 0.0 cents. An empty or zero frequency is an error, not Infinity.