A0 Frequency in Hz
A0 is 27.50 Hz in twelve tone equal temperament with A4 at the standard 440 Hz. It is MIDI note 21, piano key 1 of 88, and its wavelength in room-temperature air is about 12.47 m.
A0 sits in the sub-bass, below the lowest note of almost every acoustic instrument. Every number on this page is calculated from one formula rather than copied from a table, so the values here agree exactly with the full note frequency chart and with what the chromatic tuner reads back from your microphone.
How is the frequency of A0 calculated?
Twelve tone equal temperament divides the octave into twelve equal ratios. Each semitone multiplies the frequency by the twelfth root of two, about 1.059463, so twelve semitones multiply it by exactly two and you land one octave higher. Fix one note as the anchor and the rest follow. The anchor is A4, and the modern standard puts it at 440 Hz, which is what ISO 16 specifies as the standard tuning frequency.
Written as a formula, the frequency of MIDI note m is 440 x 2^((m - 69) / 12), because A4 is MIDI note 69. For A0 that is 440 x 2^((21 - 69) / 12) = 440 x 2^(-4.0000) = 27.50 Hz. That is 48 semitones below the reference.
What is A0 at other tuning references?
A4 at 440 Hz is the standard, not a law. Historical pitch was lower, and plenty of modern ensembles tune a little higher because a raised pitch is often described as sounding brighter in a large hall. Rebasing the same formula gives these values for A0.
| Reference A4 | A0 in Hz | Difference | Context |
|---|---|---|---|
| 415 Hz | 25.94 | -1.56 Hz | Baroque pitch, roughly a semitone below modern A |
| 432 Hz | 27.00 | -0.50 Hz | Alternative tuning used by some players, not a standard |
| 435 Hz | 27.19 | -0.31 Hz | Diapason normal, the 19th century French standard |
| 440 Hz | 27.50 | reference | Modern concert pitch, the ISO 16 standard |
| 442 Hz | 27.63 | +0.13 Hz | Common in many central European orchestras |
| 443 Hz | 27.69 | +0.19 Hz | Used by some ensembles that want a brighter sound |
Changing the reference multiplies every note by the same ratio, so intervals stay identical and only the absolute pitch moves.
What is A0 in every octave?
Moving up an octave doubles the frequency and moving down halves it, which is the one relationship in tuning that needs no calculator. Here is A across the full chart range.
| Note | Hz | MIDI | Piano key |
|---|---|---|---|
| A0 | 27.50 | 21 | 1 |
| A1 | 55.00 | 33 | 13 |
| A2 | 110.00 | 45 | 25 |
| A3 | 220.00 | 57 | 37 |
| A4 | 440.00 | 69 | 49 |
| A5 | 880.00 | 81 | 61 |
| A6 | 1760.0 | 93 | 73 |
| A7 | 3520.0 | 105 | 85 |
| A8 | 7040.0 | 117 | - |
What are the harmonics of A0?
A real instrument playing A0 does not produce 27.50 Hz alone. It produces a fundamental plus a series of whole-number multiples, and the balance between them is what makes a violin sound different from a flute on the same note. The harmonics land close to familiar notes but not exactly on them, because the harmonic series is built on simple ratios while equal temperament is not.
| Harmonic | Hz | Nearest note | Cents off |
|---|---|---|---|
| 1 (fundamental) | 27.50 | A0 | 0 |
| 2 | 55.00 | A1 | 0 |
| 3 | 82.50 | E2 | +2 |
| 4 | 110.00 | A2 | 0 |
| 5 | 137.50 | C#3 | -14 |
| 6 | 165.00 | E3 | +2 |
| 7 | 192.50 | G3 | -31 |
| 8 | 220.00 | A3 | 0 |
The seventh harmonic is the famous outlier: it lands roughly 31 cents flat of the nearest equal-tempered note, which is why it sounds bluesy rather than in tune.
How do I tune to A0?
Play the note and watch the cents readout rather than the hertz readout, because cents are proportional and hertz are not. A tuner rounds to the nearest note, so anything from 26.72 Hz to 28.31 Hz is read as A0: that is the band fifty cents either side. Most players aim to land inside five cents, which for A0 is roughly 27.42 Hz to 27.58 Hz.
Use the tone generator to play 27.50 Hz as a reference and match it by ear, or open the chromatic tuner and let your microphone do the measuring. Its neighbours are G#0 at 25.96 Hz below and A#0 at 29.14 Hz above, one semitone in each direction.
What intervals are built on A0?
| Interval above A0 | Note | Hz | Ratio |
|---|---|---|---|
| Minor third | C1 | 32.70 | 1.1892 |
| Major third | C#1 | 34.65 | 1.2599 |
| Perfect fourth | D1 | 36.71 | 1.3348 |
| Perfect fifth | E1 | 41.20 | 1.4983 |
| Octave | A1 | 55.00 | 2.0000 |
Ratios are the equal-tempered values. Pure just intonation would put the perfect fifth at exactly 1.5 and the major third at 1.25, which is why equal temperament thirds sound slightly wide.
What is the wavelength of A0?
Sound travels at roughly 343 m/s in dry air at 20 degrees Celsius, so a 27.50 Hz wave is about 12.473 m long, or 1247.3 cm. That matters for room acoustics: a wavelength that fits neatly between two parallel walls builds a standing wave, and it is also why a pipe or string producing A0 has the physical length it does. The speed of sound changes with temperature, so treat the figure as a good approximation rather than a constant.
Frequently asked questions
What is the frequency of A0 in Hz?
A0 is 27.50 Hz in twelve tone equal temperament with A4 set to the standard 440 Hz. The value comes from the formula 440 x 2^((m - 69) / 12), where m is the MIDI note number, which is 21 for A0.
What MIDI note number is A0?
A0 is MIDI note 21 when middle C is written C4, the scientific pitch notation used on this site. Software that follows the Yamaha convention labels the same MIDI number A-1, one octave lower in name only. The pitch you hear is identical either way.
How far apart are A0 and the note a semitone above it?
One semitone up from A0 is 29.14 Hz, a difference of 1.64 Hz. Every semitone is the same ratio, the twelfth root of two or about 1.059463, so the gap in hertz grows as you move higher up the keyboard.
What is A0 at A4 = 442 Hz?
Rebasing the same formula on 442 Hz gives 27.63 Hz for A0, about 0.13 Hz higher than the 440 Hz value. Raising the reference shifts every note by the identical ratio, roughly 7.85 cents from 440 to 442.
Where is A0 on a piano?
A0 is key 1 of the 88 keys, counting from A0 at the far left. It is a white key and sits in the sub-bass, below the lowest note of almost every acoustic instrument.