Select a musical note and octave to see its frequency in Hz and MIDI note number.
Equal temperament divides an octave into 12 equal semitones. The frequency of any note is calculated as f = 440 * 2^((n - 69) / 12), where n is the MIDI note number (A4 = 69). Each octave doubles the frequency: A4 = 440 Hz, A5 = 880 Hz, A3 = 220 Hz.
MIDI note numbers run from 0 (C-1) to 127 (G9). Middle C is C4, MIDI note 60, at approximately 261.63 Hz.
To find a note's frequency in equal temperament, use f = 440 x 2^((n - 69) / 12), where n is the note's MIDI number and 69 is A4's MIDI number tuned to 440 Hz. Example: A5 (MIDI 81) = 440 x 2^((81-69)/12) = 880 Hz, exactly one octave above A4.
440 Hz -- MIDI note 69, the standard tuning reference this calculator uses (f = 440 x 2^((69-69)/12) = 440).
261.63 Hz -- MIDI note 60, calculated as f = 440 x 2^((60-69)/12).
880 Hz -- exactly double 440 Hz, since every octave doubles a note's frequency.
220 Hz -- exactly half of 440 Hz, the same doubling rule applied downward.
Multiply by 2^(1/12), about 1.0595. One semitone above A4 (A#4/Bb4) is 440 x 1.0595 = 466.16 Hz.
329.63 Hz -- MIDI note 64, calculated as f = 440 x 2^((64-69)/12).
523.25 Hz -- MIDI note 72, exactly double C4's 261.63 Hz.
MIDI numbering starts at C-1 = 0 and counts up chromatically; A is 9 semitones above C, so A4 (the octave starting at C4 = 60) lands on 60 + 9 = 69. It isn't a separately chosen value -- it falls out of the standard 128-note MIDI scale.
| Note | MIDI number | Frequency (Hz) |
|---|---|---|
| A3 | 57 | 220.00 |
| C4 (middle C) | 60 | 261.63 |
| E4 | 64 | 329.63 |
| G4 | 67 | 392.00 |
| A4 | 69 | 440.00 |
| C5 | 72 | 523.25 |
| A5 | 81 | 880.00 |
440 Hz for A4 is the international reference pitch defined by ISO 16, adopted so orchestras, instrument makers, and tuners worldwide agree on the same tuning standard. Some ensembles use slightly different reference pitches (like 442 Hz) for stylistic reasons, but 440 Hz is the default most tools assume.
Doubling the frequency is the physical definition of an octave — A5 at 880 Hz sounds like the "same" note as A4 at 440 Hz, just an octave higher, and A3 at 220 Hz is an octave lower. This 2x relationship holds for every note, not just A.
MIDI note numbers are a standard way electronic instruments and software identify pitches, running from 0 (a very low C) to 127 (a very high G). Middle C is MIDI note 60, and each number up or down shifts the pitch by one semitone.
In equal temperament, C# and Db (or any enharmonic pair) share the same MIDI number and therefore the same frequency, because the 12-tone system treats them as the identical pitch even though they are spelled differently in sheet music depending on context.
This calculator uses 12-tone equal temperament with A4 = 440 Hz, the tuning standard for most modern instruments; other tuning systems (just intonation, historical temperaments, or a different reference pitch) will give slightly different frequencies for the same note name.
Sources: ISO 16 (A440 표준 피치)
Last updated: 2026-08-24