Note ↔ Frequency Converter
Convert any musical note to its frequency in hertz and back, with a live piano keyboard, MIDI number, and cents-off readout. Adjustable A4 tuning.
A4 440.00 Hz
MIDI 69
How a note becomes a frequency
Modern instruments use twelve-tone equal temperament: the octave is split into 12 equal steps, each a frequency ratio of the twelfth root of two (≈ 1.0595). Every pitch is anchored to one reference — A4 = 440 Hz, the modern concert-pitch standard (ISO 16).
From there the frequency of any note is f = 440 · 2^((n − 69) / 12), where n is the MIDI note number (A4 = 69). Going the other way, n = 69 + 12 · log₂(f / 440).
Cents — measuring how out of tune
A cent is one hundredth of a semitone, so an octave is 1200 cents. When you type a raw frequency the tool snaps to the nearest note and shows the offset in cents: 0 means perfectly in tune, ±5 is inaudible to most listeners, and ±50 sits exactly between two notes. This is the same number a guitar tuner or strobe tuner displays.
Common reference pitches
| Note | MIDI # | Frequency (A440) | Role |
|---|---|---|---|
| C0 | 12 | 16.35 Hz | Lowest organ pedal |
| E2 | 40 | 82.41 Hz | Guitar low E (6th string) |
| A2 | 45 | 110.00 Hz | Guitar A string |
| C4 | 60 | 261.63 Hz | Middle C |
| A4 | 69 | 440.00 Hz | Concert-pitch standard |
| C8 | 108 | 4186.01 Hz | Top note of a piano |
Why the A4 reference is adjustable
440 Hz only became standard in the 20th century. Baroque ensembles often tune to A4 = 415 Hz (about a semitone lower), many orchestras push to 442 or 443 Hz for brightness, and some historical organs sit far from any of these. Changing the reference rescales every note proportionally, so the tool stays accurate for early music, orchestral, and scientific (C4 = 256 Hz) tunings alike.