Note, Frequency & MIDI Converter

Frequency equals the reference pitch times two to the power of the number of semitones from that reference, divided by twelve. With A4 at 440 hertz and MIDI 69 as A4, that is f = 440 × 2^((m − 69) ÷ 12) for MIDI note m. The inverse, going from a frequency back to a note, is m = 69 + 12 × log2(f ÷ 440). Two things follow. Doubling the frequency adds twelve to the MIDI number, which is why an octave up is always twice the hertz. And the spacing between notes is multiplicative rather than additive: the semitone from A4 to A#4 is about 26 hertz, while the same semitone three octaves down is about 3.3 hertz.

One hundredth of an equal-tempered semitone, so 1200 cents to the octave. The scale is logarithmic, which is what makes it useful: five cents is the same perceived amount of out-of-tune at the bottom of a bass as at the top of a piccolo, whereas five hertz is inaudible in one place and glaring in the other. As a rough guide, under two cents is indistinguishable, five cents is where a careful listener notices on a sustained note, ten to fifteen cents sounds slightly sour in a chord, and anything past fifty cents is closer to the next note than to the one you intended.

440 was standardised by an international conference in 1939 and confirmed as ISO 16 in 1955. Before that, pitch drifted widely by country, city, and decade: baroque ensembles commonly worked near 415, which is almost exactly a semitone below modern pitch, and nineteenth-century orchestras crept upward, reaching 450 or higher, because sharper tuning sounds brighter and orchestras compete. Many European orchestras still tune to 442 or 443 for the same reason. 432 is a modern alternative promoted on claims about natural resonance that have no support in physics or in the historical record, but it is a perfectly usable reference if you like how it sounds, and the page will happily work in it.

261.63 hertz at concert pitch, and it is MIDI note 60. The naming is the confusing part rather than the number: middle C is C4 in scientific pitch notation, the convention this page uses, where the octave number changes at C rather than at A. That is why B3 is a semitone below C4 rather than a semitone above it. Yamaha and some MIDI hardware label the same note C3, shifting every octave number by one, which is a persistent source of off-by-an-octave errors when moving material between devices. The MIDI number never moves, which is exactly why it is worth quoting.

Usually the reference pitch. Check whether the tuner is set to 440, 442, or something else, because a shift of two hertz at A4 moves everything by about eight cents. After that, temperament: this page uses equal temperament, where every semitone is the same size, while a piano tuner uses stretched octaves and a string quartet adjusts thirds and fifths by ear away from equal temperament to make chords sound cleaner. Neither is wrong. Equal temperament is a compromise that lets every key be playable, and it is the right reference for electronic instruments, samples, and anything with a fixed grid.

How long one cycle of that frequency is as it travels through air, at about 343 metres a second at room temperature. It is the number that decides how sound behaves in a physical space: a 40 hertz note is roughly 8.6 metres long, which is bigger than most rooms and why bass is uneven as you walk around, while a 10 kilohertz tone is about 34 millimetres and is absorbed by a curtain. It also explains why bass traps are large and treble panels are thin, and why loudspeaker driver spacing matters at crossover.

Because it genuinely is, and no keyboard can play it. The harmonic series is fixed by physics: harmonic seven is exactly seven times the fundamental. Equal temperament divides the octave into twelve equal steps, and no combination of those steps lands on that ratio; the nearest key is about 31 cents away, roughly a third of a semitone. Brass players know this one well, since the seventh partial is noticeably flat and has to be lipped or valved into place. The eleventh is worse, sitting almost exactly between two keys, which is the sound of a natural trumpet or a bugle playing what is often described as an out-of-tune fourth.

For a reference pitch, yes: play the tone and match it by ear, and use the beating between two tones to get closer than a single tone allows. What this page does not do is listen. There is no microphone, no pitch detection, and no meter that follows your string as you turn the peg, so it is a reference rather than a tuner. If you know the frequency you are aiming for, whether that is an open string, a drum head, or a synth oscillator, this page will tell you the note and the cents, and play it back for you to match.

They exist because the MIDI specification defines 128 note numbers and the piano only occupies 88 of them. The bottom of the range, MIDI 0 at about 8.18 hertz, is below human hearing and is genuinely inaudible as a pitch, though it is useful as a control value, as a modulation rate, or as the fundamental of a synthesised sound whose harmonics are audible even when the fundamental is not. The top of the range, MIDI 127 at about 12,544 hertz, is audible as a tone to most people under forty but is above the fundamental of any orchestral instrument.

Neither. There is no microphone request, no recording, and no network call. The conversion is arithmetic and the reference tone is generated by the Web Audio API in this tab. The audio context is only created when you press play, which is both a browser requirement and a good default, and every voice is stopped when you leave the page.