27.5Hz
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27.5 HzFrequency at A4 = 440 Hz
21MIDI note number
1Piano key (of 88)
12.47 mWavelength in air
About A0
A0 has a frequency of 27.5 Hz in standard tuning, where A4 is fixed at 440 Hz. It is MIDI note 21 and key number 1 on an 88-key piano.
A0 is the lowest key on a standard 88-key piano.
In equal temperament every semitone multiplies the frequency by the twelfth root of two (≈1.05946), so A0 is 48 semitones below A4: f = 440 × 2^((21 − 69) / 12) = 27.50 Hz. The number in the name is the octave in scientific pitch notation, which starts each octave on C.
A0 frequency in different tunings
| Tuning reference | A0 frequency |
|---|---|
| A4 = 440 Hz (ISO 16 standard) | 27.5 Hz |
| A4 = 442 Hz (many European orchestras) | 27.62 Hz |
| A4 = 443 Hz (Berlin, Vienna) | 27.69 Hz |
| A4 = 435 Hz (French diapason normal, 1859) | 27.19 Hz |
| A4 = 432 Hz (alternative tuning) | 27 Hz |
| A4 = 415 Hz (Baroque pitch) | 25.94 Hz |
| C4 = 256 Hz (scientific pitch) | 26.91 Hz |
Intervals from A0
| Interval | Note | Frequency | Ratio (just) |
|---|---|---|---|
| Minor second | A♯0 | 29.14 Hz | 16:15 |
| Major second | B0 | 30.87 Hz | 9:8 |
| Minor third | C1 | 32.7 Hz | 6:5 |
| Major third | C♯1 | 34.65 Hz | 5:4 |
| Perfect fourth | D1 | 36.71 Hz | 4:3 |
| Tritone | D♯1 | 38.89 Hz | 45:32 |
| Perfect fifth | E1 | 41.2 Hz | 3:2 |
| Minor sixth | F1 | 43.65 Hz | 8:5 |
| Major sixth | F♯1 | 46.25 Hz | 5:3 |
| Minor seventh | G1 | 49 Hz | 9:5 |
| Major seventh | G♯1 | 51.91 Hz | 15:8 |
| Octave | A1 | 55 Hz | 2:1 |
Instruments and voices that can play A0
Frequently asked questions
What is the frequency of A0?
A0 is 27.5 Hz when A4 = 440 Hz (27 Hz if A4 = 432 Hz).
What MIDI number is A0?
A0 is MIDI note number 21.
Is A0 on a piano?
Yes – it is key 1 of 88.