Note to Frequency Calculator
Convert any musical note to its pitch in hertz, or find the nearest note to a frequency.
A4 is MIDI note 69 in 12-tone equal temperament with A4 = 440 Hz.
In 12-tone equal temperament with A4 tuned to 440 Hz, a note’s frequency is f = 440 × 2^((n − 69) ÷ 12), where n is its MIDI number. So A4 (n = 69) is exactly 440 Hz and middle C, C4 (n = 60), works out to about 261.63 Hz.
How notes map to frequencies
Every musical pitch is a frequency measured in hertz (Hz) — the number of times per second the sound wave repeats. In equal temperament the octave is split into twelve equal semitones, so moving up one semitone always multiplies the frequency by the twelfth root of two (≈ 1.059463). Numbering the notes with the MIDI standard — where A4 = 69 and C4 = 60 — lets a single formula give the frequency of any pitch from a low rumble to the top of the piano.
f is frequency in Hz, n the MIDI note number, and 440 Hz the tuning of A4 (n = 69)
Worked example
Find the frequency of middle C, written C4.
- 1 Find the MIDI note number. C4 has pitch class C = 0 and octave 4, so n = 12 × (4 + 1) + 0 = 60.
- 2 Count semitones from A4. A4 is n = 69, so C4 is 60 − 69 = −9 semitones below it.
- 3 Raise 2 to that many twelfths. 2^(−9 ÷ 12) = 2^(−0.75) ≈ 0.594604.
- 4 Multiply by 440 Hz. 440 × 0.594604 ≈ 261.63 Hz — the frequency of middle C.
Frequencies of one octave (C4–B4)
Equal-tempered frequencies with A4 = 440 Hz, rounded to two decimals. The octave above doubles each value (C5 = 523.25 Hz); the octave below halves it (C3 = 130.81 Hz).
| Note | MIDI number (n) | Frequency (Hz) |
|---|---|---|
| C4 (middle C) | 60 | 261.63 |
| C♯4 | 61 | 277.18 |
| D4 | 62 | 293.66 |
| D♯4 | 63 | 311.13 |
| E4 | 64 | 329.63 |
| F4 | 65 | 349.23 |
| F♯4 | 66 | 369.99 |
| G4 | 67 | 392.00 |
| G♯4 | 68 | 415.30 |
| A4 | 69 | 440.00 |
| A♯4 | 70 | 466.16 |
| B4 | 71 | 493.88 |
Tuning standards and temperament
Why an octave doubles the frequency. Two notes an octave apart sound like the same pitch because the higher one vibrates exactly twice as fast. That 2 : 1 ratio is why the exponent divides by 12: twelve semitones multiply the frequency by 2^(12 ÷ 12) = 2, one full octave. A5 is therefore 880 Hz, double A4’s 440 Hz.
Equal temperament vs. just intonation. This calculator uses 12-tone equal temperament, where every semitone is identical so music can be played in any key. Just intonation instead tunes intervals to simple whole-number ratios (a perfect fifth as 3 : 2, for example), giving purer chords but only in one key. The two systems disagree by a few cents on most notes.
The 440 Hz standard. A4 = 440 Hz is the modern international reference, but it is a convention, not a law of physics. Many orchestras tune slightly sharper to 442 or 443 Hz for a brighter sound, and some ensembles prefer 432 Hz. Change the reference and every frequency scales with it in the same proportion.