A4 Frequency — Why 440 Hz Is the Tuning Standard
A4 — the A above middle C — is 440 Hz. That number is not a law of physics; it is an agreement. It is the note orchestras tune to, the pitch a tuner’s reference tone plays, and the anchor from which every other frequency on this site is calculated.
A4 at a glance
| Property | Value |
|---|---|
| Frequency | 440 Hz |
| MIDI note number | 69 |
| Scientific pitch notation | A4 |
| Octave below (A3) | 220.00 Hz |
| Octave above (A5) | 880.00 Hz |
| Wavelength in air at 20 °C | 0.780 m |
| Period of one cycle | 2.2727 ms |
Where 440 Hz came from
Pitch standards were all over the place for centuries. In the Baroque era, “A” could mean anything from roughly 415 Hz to 470 Hz depending on the town, the church organ, or the instrument maker. As railways and touring orchestras made musicians travel, that became a practical problem: a violin built for a low pitch could not play with an organ built for a high one.
- 1859 — France fixes A at 435 Hz (the diapason normal).
- 1939 — an international conference in London recommends 440 Hz.
- 1955 / 1975 — the International Organization for Standardization adopts it as ISO 16, which is where the modern standard comes from.
Not everyone tunes to 440
The standard is a default, not a rule, and there are good reasons to depart from it:
- 442–445 Hz — many European and American orchestras tune slightly sharp for a brighter, more projecting sound.
- 415 Hz — roughly a semitone below 440, the usual choice for historically informed Baroque performance.
- 432 Hz — often claimed online to be “natural” or more relaxing. There is no acoustic or historical evidence for those claims; it is a preference, not a standard.
None of this changes the maths. Change the reference and the whole grid slides with it: at 442 Hz, A4 is 442 Hz and middle C becomes 262.81 Hz instead of 261.63 Hz.
Why A4 matters for everything on this chart
Every frequency in the note frequency chart is derived
from A4. In twelve-tone equal temperament the formula is
f = 440 × 2(m − 69) / 12, with m the MIDI note number. Set A4 to
442 instead and the same formula shifts every note by about 7.9 cents — small, but audible to a
trained ear.
Frequently asked questions
- What is the frequency of A4?
- A4 is 440 Hz. It is the international tuning standard defined in ISO 16, and it is MIDI note 69.
- Why is A4 440 Hz and not a round number like 400?
- It is a compromise. Historical pitches ranged from about 415 to 470 Hz; 440 was adopted in 1939 because it sat within the range most instruments of the day were already built for. There is no acoustic reason it must be 440.
- Is 432 Hz better than 440 Hz?
- There is no scientific evidence that 432 Hz is more natural or more relaxing than 440 Hz. It is a listening preference. However, tuning flat does reduce string tension slightly, which some players like.
- What frequency is A4 on a piano in practice?
- Theoretically 440 Hz, but piano tuners use stretch tuning, sharpening the high notes and flattening the low ones to compensate for string inharmonicity, so the top of a piano runs slightly sharp.
- What are the A notes above and below A4?
- A3 is 220.00 Hz and A5 is 880.00 Hz. Each octave doubles the frequency.
Last updated: 2026-09-01