How note frequencies are calculated
In twelve-tone equal temperament every semitone multiplies the frequency by the twelfth root of two (about 1.0595), and every octave doubles it. Starting from the reference A4 = 440 Hz, the frequency of any note is 440 × 2(n − 69) / 12, where n is the MIDI note number. Middle C (C4, MIDI 60) is therefore 261.63 Hz.
Octave numbers
This chart uses scientific pitch notation, where middle C is C4 and the lowest note of an 88-key piano is A0 (27.5 Hz). Some software, for example certain DAWs, calls middle C "C3"; the MIDI number 60 stays the same either way.
Cents and the reverse lookup
Type any frequency into the second box to find the nearest note and how far away it is in cents. That is useful for checking feedback frequencies, room resonances or the pitch of a sample.
Wavelength
Wavelength is calculated for sound in air at about 20 °C (343 m/s). Low notes have long waves: the open low E of a bass guitar (E1, 41.2 Hz) has a wavelength of over 8 metres, which is why bass traps need to be so big.