Every musical note has an exact frequency in hertz. This calculator converts any note name and octave into its frequency using 12-tone equal temperament and an adjustable A4 reference, and also reports the MIDI note number and the wavelength in air — handy for synth design, tuning and acoustics.
How it works
Each note is first mapped to a MIDI note number, where A4 = 69 and every semitone changes the number by one. The frequency follows the equal-temperament formula:
f = ref × 2^((n − 69) / 12)
where n is the MIDI number and ref is the A4 reference (default 440 Hz). Because the exponent steps by 1/12 per semitone, an octave (12 semitones) exactly doubles the frequency. The wavelength is then 343 ÷ f metres, using the speed of sound in air at about 20°C.
Example
For A4 with the standard 440 Hz reference:
| Value | Result |
|---|---|
| MIDI number | 69 |
| Frequency | 440 Hz |
| Wavelength | 343 ÷ 440 ≈ 0.78 m |
Move up an octave to A5 (MIDI 81) and the frequency doubles to 880 Hz. Move down a semitone from A4 to G♯4 (MIDI 68) and the frequency is 440 × 2^(−1/12) ≈ 415.30 Hz. Change the reference to 415 Hz for baroque tuning and every note shifts accordingly.
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